Seres Therapeutics reported a net loss of $16.29 million

Cover page 2 changes

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For the quarterly period ended June September 30, 2026

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As of August 3, November 2, 2026, the registrant h ad 9,941,261 10,195,031 shares of common stock, $0.001 par value per share, outstanding.

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Forward-looking statements 1 change

This Quarterly Report on Form 10-Q, or the Quarterly Report, contains forward-looking statements. We intend such forward-looking statements to be covered by the safe harbor provisions for forward-looking statements contained in Section 27A of the Securities Act of 1933, as amended (the "Securities Act"), and Section 21E of the Securities Exchange Act of 1934, as amended (the "Exchange Act"). All statements other than statements of historical facts contained in this Quarterly Report, including without limitation statements regarding our future results of operations and financial position including our projected cash runway, our requirement for additional funding, our business strategy, including potential strategic partnership efforts, our prospective products, the design, timing and results of our clinical studies and data readouts, product approvals, communications with, feedback from, or submissions to the FDA, research and development costs, timing and likelihood of success, our ability to continue as a going concern, our ability to maintain compliance with any applicable Nasdaq listing requirements, executive and director transition matters, plans and objectives of management for future operations and future results of anticipated products, the expected receipt of the Milestone Termination Payment (as defined herein) or the timing of any of the foregoing, are forward-looking statements. These statements involve known and unknown risks, uncertainties and other important factors that may cause our actual results, performance or achievements to be materially different from any future results, performance or achievements expressed or implied by the forward-looking statements.

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Financial statements 30 changes

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Stockholders' equity: (deficit):

Preferred stock, $ 0.001 par value; 10,000,000 shares authorized at June September 30, 2026 and December 31, 2025; no shares issued and outstanding at June September 30, 2026 and December 31, 2025

Common stock, $ 0.001 par value; 360,000,000 shares authorized at June September 30, 2026 and December 31, 2025; 9,827,569 10,049,501 and 9,556,466 shares issued and outstanding at June September 30, 2026 and December 31, 2025, respectively

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Net income (loss) and comprehensive income (loss)

Net (loss) income and comprehensive (loss) income

Net income (loss) income per share attributable to common stockholders - basic ( 2.27 1.63 ( 1.59 3.24

Net income (loss) income per share attributable to common stockholders - diluted ( 2.27 1.63 ( 1.59 3.24

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[1] Includes $ 0 , $ 0 , $ 3,490 2,028 , and $ 9,799 11,827 for the three and six nine months ended June September 30, 2026 and 2025 related to reimbursement received from SPN (related party) for transition services provided by the Company.

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CONDENSED CONSOLIDATED STATEMENTS OF STO CKHOLDERS' EQUITY (DEFICIT)

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Issuance of common stock upon vesting of RSUs, net of tax withholdings

Issuance of common stock from at the market equity offering, net of issuance costs of $ 106

Issuance of common stock in connection with lease termination

Issuance of common stock under ESPP

Stock-based compensation expense

Balance at September 30, 2026

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Issuance of common stock upon exercise of stock options

Issuance of common stock upon vesting of RSUs, net of tax withholdings

Issuance of common stock under ESPP

Stock-based compensation expense

Balance at September 30, 2025

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Six Months Ended June 30,

Nine Months Ended September 30,

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Gain on sale of VOWST business

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Prepaid expenses and other current and other non-current assets ( 496 221 ( 1,887 2,402

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Accrued expenses and other current and long-term liabilities ( 103 2,031 ( 4,794 4,161

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Proceeds from VOWST milestone termination payment

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Proceeds from exercise of stock options

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Net (decrease) increase in cash, cash equivalents, and restricted cash ( 36,597 33,770

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Property and equipment purchases included in accounts payable and accrued expenses

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Equity issued in connection with lease termination

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Note 1: Nature of the Business and Basis of Presentation 8 changes

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The Company's initial focus in oncology is in two areas: (1) to prevent bacterial bloodstream infections ("BSIs"), including those that can harbor antimicrobial resistance ("AMR"), as well as other pathogen-associated negative clinical outcomes in patients undergoing allogeneic hematopoietic stem cell transplantation ("allo-HCT"), and (2) to treat prevent immune checkpoint-related enterocolitis ("irEC"), a frequent and often severe side effect of immune checkpoint inhibitor ("ICI") therapy.

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SER-155 is also being evaluated in 15 patients with moderate-to-severe (Grade 2- 3) developed to prevent irEC, through an investigator-sponsored trial ("IST"), with Memorial Sloan Kettering the aim of enabling patients to continue their ICI cancer Center, with whom therapy and avoid the Company has had a decade-long collaboration. need for immunosuppressive steroid treatment. irEC is among the most frequent and severe immune-related adverse events ("irAEs"), in recipients of ICI therapy, and can be observed in up to 50% of ICI patients, with rates varying based on cancer drug and treatment regimen. Moderate to severe (Grade 2-3) irEC occurs in approximately 25% of all ICI therapy recipients in the United States. Clinical guidelines for patients who experience Grade 2 or higher irEC require treatment, which could include halting of ICI cancer treatment and initiating a systemic immunosuppressive corticosteroid, a biologic, or both, which are associated with clinically meaningful toxicities and other negative consequences, including systemic immunosuppression, increased infection risk, metabolic complications, and potential interference with cancer treatment. SER-155 was evaluated in 15 patients with moderate-to-severe irEC through an investigator-sponsored trial ("IST"), with Memorial Sloan Kettering Cancer Center, with whom the Company has had a decade-long collaboration. Top-line data from the IST showed that 80% of SER-155 recipients achieved an immunosuppressive-free clinical response of diarrhea (a primary symptom of irEC) at Day 15, which met the primary endpoint, defined as at least a 1-grade improvement in diarrhea symptoms without immunosuppressive therapy (i.e., a corticosteroid and/or biologic immunomodulator drug). All 12 participants with immunosuppressive-free clinical response at Day 15 had the same, or better, diarrhea grade at Day 43. SER-155 was well tolerated, with no drug-related serious adverse events observed. The Company is progressing plans for a next study evaluating SER-155 for the prevention of irEC in patients initiating ICI cancer therapy, which is expected to include a dose optimization component. The Company expects to submit the study design to the FDA in the first quarter of 2027.

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On February 12, 2026, the Company announced that it implemented cost reduction actions, including reducing its workforce by approximately 30 %. During the six nine months ended June September 30, 2026 , the Company incurred approximately $ 914 in restructuring costs, primarily related to severance costs, of which $ 718 was paid in the first quarter of 2026, and the remainder of these costs were paid out in the second quarter of 2026.

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The accompanying condensed consolidated financial statements have been prepared on a basis which assumes that the Company will continue as a going concern and which contemplates the realization of assets and satisfaction of liabilities and commitments in the normal course of business. As of June September 30, 2026, the Company had an accumulated deficit of $ 987,735 1,004,021 and cash and cash equivalents of $ 15,594 17,528 .

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Primarily as a result of the costs associated with the discovery and development of novel live biotherapeutics, the Company incurred a net loss from operations of $ 42,180 58,332 for the six nine months ended June September 30, 2026 . The Company expects that its operating losses and negative cash flows will continue for the foreseeable future. Based on the Company's currently available cash resources and the $ 12,500 Milestone Termination Payment (as defined below in Note 3, Discontinued Operations and TSA ) received from SPN (as defined below) in July 2026, and the expected receipt of the additional $ 12,500 Milestone Termination Payment due from SPN in October 2026 , current and forecasted level of operations, and forecasted cash flows for the 12-month period subsequent to the date of issuance of these condensed consolidated financial statements, the Company will require additional funding to support its ongoing operations and meet its obligations as they come due. These conditions raise substantial doubt about the Company's ability to continue as a going concern.

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The accompanying unaudited condensed consolidated financial statements as of June September 30, 2026 and for the six three and nine months ended June September 30, 2026 and 2025 have been prepared by the Company, pursuant to the rules and regulations of the Securities and Exchange Commission (the "SEC") for interim financial statements. Certain information and footnote disclosures normally included in financial statements prepared in accordance with accounting principles generally accepted in the United States of America ("GAAP") have been condensed or omitted pursuant to such rules and regulations. However, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited condensed consolidated financial statements should be read in conjunction with the Company's audited consolidated financial statements and the notes thereto for the year ended December 31, 2025 included in the Company's annual report on Form 10-K for the fiscal year ended December 31, 2025, which was filed with the SEC on March 12, 2026 (the "Annual Report").

The unaudited condensed consolidated interim financial statements have been prepared on the same basis as the audited consolidated financial statements. The condensed consolidated balance sheet at December 31, 2025 was derived from audited annual financial statements, but does not contain all of the footnote disclosures from the annual financial statements. In the opinion of management, the accompanying unaudited interim condensed consolidated financial statements contain all adjustments which are necessary for a fair statement of the Company's financial position, results of operations, and cash flows for the periods presented. Such adjustments are of a normal and recurring nature. The results of operations for the three and six nine months ended June September 30, 2026 are not necessarily indicative of the results of operations that may be expected for the year ending December 31, 2026.

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On April 21, 2025, the Company effected a 1-for-20 reverse stock split of the Company's common stock. All share and per share amounts in the financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse stock split. T he reverse stock split had no impact on the number of authorized shares or the par value of preferred and common stock. Therefore, the Company reclassified an amount equal to the reduction in the number of shares of common stock at par value to additional paid-in capital. No fractional shares were issued in connection with the reverse split, and stockholders who would otherwise be entitled to receive a fractional share instead received a cash payment equal to the fraction of a share of common stock in lieu of such fractional share. Proportionate adjustments were made to the number of shares authorized under the Company's equity incentive plans, the number of shares subject to any award or purchase right under the Company's equity incentive plans, and the exercise price or purchase price with respect to any stock option award or purchase right under the Company's equity incentive plans. See Note 7, Stockholders' Equity Common Stock and Stock-Based Awards , for further information.

Note 2: Summary of Significant Accounting Policies 3 changes

The significant accounting policies and estimates used in preparation of the unaudited condensed consolidated financial statements are described in the Company's audited financial statements as of and for the year ended December 31, 2025, and the notes thereto, which are included in the Annual Report. There have been no material changes to the Company's significant accounting policies during the six nine months ended June September 30, 2026.

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The Company held restricted cash of $ 2,243 3,136 and $ 8,668 as of June September 30, 2026 and December 31, 2025, respectively, which represents cash held for the benefit of the landlords for the Company's leases. As of September 30, 2026 the Company has classified $ 2,363 of restricted cash within prepaid expenses and other current assets as the terms of the relative underlying lease is less than one year and $ 773 of restricted cash as long-term on its condensed consolidated balance sheets as the terms of the underlying leases are greater than one year.

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Restricted cash, non-current

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Note 3: Discontinued Operations and TSA 7 changes

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The Company also entered into a Transition Services Agreement ("TSA") with Nestlé Enterprises S.A., ("NESA"), an affiliate of SPN, in connection with the Transaction, through which the Company provided certain manufacturing services until December 31, 2025, and other transition services, for the duration specified in the schedule to the TSA for each service. The Company's obligations under the TSA are now complete. For the three and six nine months ended June September 30, 2026 and 2025, the Company recognized $ 0 , $ 0 , $ 3,490 2,028 , and $ 9,799 11,827 respectively, of TSA reimbursement income in other income in the Company's condensed consolidated statements of operations and comprehensive (loss) income. For the three and six nine months ended June September 30, 2026 and 2025, the Company incurred $ 0 , $ 0 , $ 1,689 736 , and $ 5,216 5,952 , respectively, of expenses related to manufacturing services and $ 0 , $ 0 , $ 1,261 76 , and $ 3,509 3,585 , respectively, of TSA labor and passthrough expenses to support the transition services, including finance and accounting, information technology, human resources, operations, and other services.

For the three and nine months ended September 30, 2026 and 2025, $ 0 , $ 0 , $ 1,756 , and $ 13,743 , respectively, was billed to NESA related to transition services performed by the Company, and the Company received $ 0 , $ 360 , $ 2,212 , and $ 90,385 from

For the three and six months ended June 30, 2026 and 2025, $ 0 , $ 0 , $ 4,585 , and $ 11,987 , respectively, was billed to NESA related to transition services performed by the Company, and the Company received $ 0 , $ 360 , $ 6,464 , and $ 63,173 from NESA during the periods, respectively. The payments received in the first quarter of 2025 included the installment payment of $ 50,000 received in January 2025 and the installment payment of $ 25,000 received in July 2025 which was were conditioned on the Company's material compliance with obligations under the TSA. The installment payment was payments were recognized in Gain on sale of VOWST Business within continuing operations in the Company's condensed consolidated statements of operations and comprehensive income for the six three and nine months ended June September 30, 2025 as the gain was realizable.

The Company has estimated costs associated with certain accrued liabilities due to SPN - related party as a loss contingency in accordance with ASC 450, Contingencies. These contingent liabilities are presented as Accrued Liabilities due to SPN - related party from continuing operations on the condensed consolidated balance sheet as of June September 30, 2026 and December 31, 2025 and consist of the following (in thousands):

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The contingent liabilities accrued on the Company's condensed consolidated balance sheet are remeasured at each reporting period based on i) cash payments made by the Company to reduce the accrued liabilities due to SPN - related party and ii) revised estimates of the total remaining liabilities due to SPN - related party. For the three and six nine months ended June September 30, 2026 and 2025 , the Company recognized a gain on sale of VOWST Business of $ 0 , $ 0 , $ 185 2,222 , and $ 2,366 4,588 , respectively, as a result of the change in accrued liabilities due to SPN - related party.

On June 2, 2026, the Company entered into Amendment No. 1 (the "APA Amendment") to the Purchase Agreement. Pursuant to the Purchase Agreement, SPN agreed to pay the Company certain one-time contingent milestone payments during the Milestone Period (as defined in the Purchase Agreement) if and following the first time that the worldwide annual net sales of the Product (as defined in the Purchase Agreement) reached the following thresholds: (i) a $ 125,000 milestone payment upon the first calendar year in which annual worldwide net sales of the Product equal or exceed $ 400,000 (the "Second Sales Milestone Payment"); and (ii) a $ 150,000 milestone payment upon the first calendar year in which annual worldwide net sales of the Product equal or exceed $ 750,000 (the "Third Sales Milestone Payment"). Pursuant to the Purchase Agreement, the Prepaid Milestone (as defined in the Purchase Agreement) accrued interest during the Milestone Period, which would reduce the Second Sales Milestone Payment to Seres if and when it became payable (the "Milestone Interest Payments"). Pursuant to the APA Amendment, the parties agreed to terminate SPN's obligations with respect to the Second Sales Milestone Payment, the Third Sales Milestone Payment and eliminate the Milestone Interest Payments that would have been due from the Company to SPN. In consideration for the foregoing, SPN agreed to a one-time payment to the Company of $ 25,000 (the "Milestone Termination Payment"), which amount is payable in two equal installments of $ 12,500 , with the first installment paid on July 1, 2026 and the second installment payable paid on October 1, 2026.

As there are no conditions under which the Company would be required to repay the $ 25,000 , or any portion of the interest that was accruing on the Prepaid Milestone under the Purchase Agreement, and there are no circumstances under which SPN would be relieved of its obligation to make this payment to the Company, the gain is was considered realizable in June 2026 when the APA Amendment was signed. The Company recorded a Gain on Sale of VOWST Business within continuing operations of $ 25,000 during the three nine months ended June September 30, 2026 because the amount is fixed consideration and is no longer contingent on a future event. As of June September 30, 2026 and December 31, 2025, the Company had $ 25,000 12,500 and $ 360 respectively, in accounts receivable due from SPN - related party in the Company's condensed consolidated balance sheets.

Note 4: Property and Equipment, Net 1 change

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Depreciation and amortization expense was $ 670 481 , $ 1,451 1,932 , $ 1,059 1,044 , and $ 2,135 3,179 for the three and six nine months ended June September 30, 2026 and 2025, respectively. During the three and six nine months ended June September 30, 2026 , the Company disposed of certain assets with a cost basis of $ 3,155 28,670 and $ 31,825 and a net book value of $ 386 2,318 and $ 2,704 , primarily related to leasehold improvements that were impaired in connection with the Termination Agreement as defined in Note 6, Leases , as well as assets that were returned to the landlord in connection with the Lease Amendment as defined in Note 6, Leases . The Company did no t dispose of any assets during the three and six nine months ended June September 30, 2025 .

Note 6: Leases 14 changes

The Company leases real estate, primarily laboratory, office and manufacturing space. The Company's leases have remaining terms ranging from approximately three less than one year to ten years . Certain leases include One or more options of the Company's leases includes an option to renew, exercisable at the Company's sole discretion, with renewal terms that can extend the lease from approximately one year for up to ten years . The Company previously evaluated the renewal options in its leases to determine if it was reasonably certain that the renewal option would be exercised. Given the Company's current business structure, uncertainty of future growth, and the associated impact to real estate, the Company concluded that it is not reasonably certain that any renewal options would be exercised. Therefore, the operating lease assets and operating lease liabilities only contemplate the initial lease terms. All the Company's leases qualify as operating leases.

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During the three nine months ended June September 30, 2026, the Company identified an indicator of impairment relating to the Early Termination Premises, as the Company is not planning to use the Early Termination Premises; additionally, the Company is prohibited under the lease from marketing the Early Termination Premises for sublease. The Company determined that this represents a significant adverse change in the extent in which the long-lived asset was being used. The Company concluded that the carrying value of the Early Termination Premises, which represents a single asset group, was not recoverable as there are zero future cash flows that are expected to be generated from the assets within the asset group. The Company recognized an impairment loss of $ 5,807 , consisting of $ 5,574 on the operating lease right-of-use asset and $ 233 on the leasehold improvements, in the three and six nine months ended June September 30, 2026. The impairment loss is disaggregated in the accompanying condensed consolidated statements of operations and comprehensive income (loss) as a separate line, impairment of long-lived assets.

On July 31, 2026, the Company entered into the Third Amendment to Lease and Termination Agreement (the "Termination Agreement") with BMR-Sidney Research Campus LLC, a Delaware limited liability company (the "Sidney Street Landlord"), the purpose of which is to effect an early termination of one of the Company's leases, reducing the Company's ongoing annual facilities costs and materially eliminating the Company's primary restoration obligations. The Termination Agreement amends the Lease Agreement, dated November 11, 2015, by and between the Company and the Sidney Street Landlord (as amended, the "Sidney Street Lease"), pursuant to which the Company leases approximately 68,636 rentable square feet of office, laboratory, and manufacturing space located at 200 Sidney Street, Cambridge, Massachusetts (the "Existing Premises").

The Termination Agreement provides for (i) effective as of August 1, 2026, the surrender by the Company to the Sidney Street Landlord of an aggregate area of approximately 21,295 rentable square feet of the Existing Premises (the "Sidney Early Termination Premises"), and (ii) the revision of the expiration date of the term of the Lease with respect to the remaining 47,341 rentable square feet of the Existing Premises (the "Renewal Premises") from January 13, 2031 to December 31, 2026. As consideration for the Termination Agreement, the Company agreed to pay the following: (i) an increase to the letter of credit held by the Sidney Street Landlord in the amount of $ 2,238 (for a total letter of credit of $ 3,638 ), which the Sidney Street Landlord will apply toward rent and operating expenses through the term of the Renewal Premises, (ii) on or before January 4, 2027, a termination payment of $ 3,850 , and (iii) an equity issuance of shares of the Company's common stock valued at $ 500 (the "Share Issuance").

On July 31, 2026, in connection with the Share Issuance, the Company entered into a Stock Issuance Agreement (the "Stock Issuance Agreement") with the Sidney Street Landlord, pursuant to which the Company agreed to issue 103,520 shares of Common Stock (the "Shares") to the Sidney Street Landlord at a price per share of $ 4.83 , which was the closing price of the Common Stock on the Nasdaq Global Select Market ("Nasdaq") on July 31, 2026. The Shares are registered under the Company's registration statement on Form S-3 (File No. 333-273794), pursuant to a prospectus supplement, dated July 31, 2026 and the accompanying base prospectus, dated August 15, 2023, and were freely tradeable when issued. The Stock Issuance Agreement contains customary representations and warranties of the Company and the Sidney Street Landlord and customary conditions to the share issuance.

The reduction of the lease term and associated lease payments for the Renewal Premises does not grant the Company an additional right-of-use, so it is therefore accounted for as a modification to the existing lease and not a new contract separate from the existing contract. Accordingly, the Company remeasured the lease liability to account for the remaining lease payments, inclusive of the letter of credit, the future termination payment, and the equity issuance, over the remaining lease term and decreased the associated lease liability and right-of-use asset by $ 18,884 .

During the three months ended September 30, 2026, the Company identified an indicator of impairment relating to the Renewal Premises, as the Company fully exited the Renewal Premises concurrent with the Termination Agreement; additionally, given the reduced lease term of five months from the date the Termination Agreement was executed, the Company is unable to sublease the Renewal Premises. The Company determined that this represents a significant adverse change in the extent to which the long-lived asset was being used. The Company concluded that the carrying value of the Renewal Premises, which represents a single asset group, was not recoverable and the fair value of the asset group is zero. The Company recognized an impairment loss of $ 5,705 , consisting of $ 3,654 on the operating lease right-of-use asset and $ 2,051 on the leasehold improvements, in the three and nine months ended September 30, 2026. The impairment loss is disaggregated in the accompanying condensed consolidated statements of operations and comprehensive income (loss) as a separate line, impairment of long-lived assets.

Concurrent with the Termination Agreement, the Company assigned its existing sublease for the Sidney Early Termination Premises to the Sidney Street Landlord. As a result, the Company concluded that the remaining straight-line sublease receivable would not be realized and recognized a loss of $ 469 upon termination of the sublease, which is included within other income (expense) in the accompanying condensed consolidated statements of operations and comprehensive income (loss).

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2026 (remaining 6 months)

Sublease (income) expense

During the three and six nine months ended June September 30, 2026 and 2025, the Company made cash payments for operating leases of $ 14,166 2,584 , $ 19,147 21,731 , $ 4,837 , and $ 9,715 14,552 , respectively.

As of June September 30, 2026, future payments of operating lease liabilities are as follows (in thousands):

2026 (remaining 3 months)

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As of June September 30, 2026, the weighted average remaining lease term was 7.10 8.49 years and the weighted average incremental borrowing rate used to determine the operating lease liability was 11.4 9.5 % . As of June September 30, 2025, the weighted average remaining lease term was 6.52 6.29 years and the weighted average incremental borrowing rate used to determine the operating lease liability was 13 % .

Note 7: Common Stock and Stock-Based Awards 13 changes

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Additionally, at the 2025 Annual Meeting, stockholders approved the Seres Therapeutics, Inc. 2025 Incentive Award Plan (the "2025 Plan") as an amendment and restatement of the Seres Therapeutics, Inc. 2015 Incentive Award Plan (the "2015 Plan"). The amendment, among other things, authorized the issuance of 2,230,243 shares of the Company's common stock for awards under the 2025 Plan, which includes 1,750,493 shares previously authorized for issuance under the 2015 Plan plus an increase of 479,750 shares (in each case, which amounts reflect the 1-for- 20 reverse stock split), and extended the term of the 2025 Plan to March 3, 2035, the tenth anniversary of the approval of the 2025 Plan by the Company's board of directors. On July 8, 2026, at the Company's reconvened 2026 annual meeting of stockholders (the "2026 Annual Meeting"), stockholders approved an amendment and restatement of the 2025 Plan to increase the number of authorized shares available for issuance under the 2025 Plan by 900,000 .

On May 21, 2021, the Company entered into a Sales Agreement (the "Sales Agreement") with Cowen and Company, LLC ("Cowen") to sell shares of the Company's common stock, with aggregate gross sales proceeds of up to $ 150,000 , from time to time, through an "at the market" equity offering program ("ATM") under which Cowen acts as sales agent. During the three and six nine months ended June September 30, 2026, the Company sold 232,286 113,643 and 258,082 371,725 shares of common stock under the Sales Agreement, at an average price of approximately $ 7.99 7.45 and $ 8.78 8.37 per share, respectively, raising aggregate net proceeds of approximately $ 1,715 741 and $ 2,115 2,856 after deducting an aggregate commission of approximately 3 % and other issuance costs. During the six nine months ended June September 30, 2025, the Company sold 54,806 shares of common stock under the Sales Agreement, at an average price of approximately $ 18.60 per share, raising aggregate net proceeds of approximately $ 996 after deducting an aggregate commission of approximately 3 % and other issuance costs. On August 13, 2026, the Company entered into a Sales Agreement (the "2026 Sales Agreement") with TD Securities (USA) LLC ("TD Cowen") to sell shares of the Company's common stock, with aggregate gross sales proceeds of up to $ 100,000 , from time to time, through an at the market equity offering program under which TD Cowen acts as sales agent. During the three months ended September 30, 2026, no shares were sold under the 2026 Sales Agreement.

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Outstanding as of June September 30, 2026

Vested or expected to vest as of June September 30, 2026

Options exercisable as of June September 30, 2026

The weighted average grant date fair value of stock options granted during the six months ended June 30, 2026 and the three and six nine months ended June September 30, 2026 and 2025 was $ 7.79 6.34 , $ 10.18 7.43 , $ 10.34 , and $ 13.49 13.30 per share, respectively. The Company did no t grant any stock options during the three months ended June 30, 2026.

During 2024, the Company granted stock options to certain executives for the purchase of an aggregate of 127,500 shares of common stock. These awards will vest only to the extent that the 30-day trailing simple average public market closing price of the Company's common stock reaches certain price thresholds. These awards have an exercise price of $ 22.00 and vest and become exercisable when the market conditions are satisfied or, if later, on the first anniversary of the grant date. These awards expire 10 years from the date of grant. The fair value of these market-based stock options was estimated using a Monte Carlo valuation method. During the three and six nine months ended June September 30, 2026 and 2025 , the Company recognized $ 82 83 , $ 248 331 , $ 188 190 and $ 261 451 , respectively, of compensation expense related to these awards.

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Unvested restricted stock units as of June September 30, 2026

During the three and six nine months ended June September 30, 2026 and 2025, the Company granted 0 , 173,385 , 0 68,596 , and 11,250 79,846 RSUs, respectively. During the three and six nine months ended June September 30, 2026 and 2025 , the Company did no t grant any PSUs.

In February 2026, the Company issued retention awards to non-executive employees of the Company in the form of RSUs covering 173,385 shares of common stock with a grant date fair value of $ 8.74 per share. The retention RSUs will become were fully vested on September 30, 2026 subject to the employee's continued employment with the Company through such date. The compensation expense associated with these awards is being recognized ratably over the vesting period. For the three and six nine months ended June September 30, 2026, the Company recognize d $ 523 530 and $ 741 1,271 i n compensation expense with respect to the retention RSUs.

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Six Months Ended June 30,

Three Months Ended June September 30,

Nine Months Ended September 30,

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Note 8: Net (Loss) Income per Share 10 changes

Six Months Ended June 30,

Basic and diluted net income (loss) income per share attributable to common stockholders was calculated as follows (in thousands, except share and per share data):

Three Months Ended June September 30,

Nine Months Ended September 30,

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Income (Loss) income attributable to common stockholders - basic

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Net income (loss) income per share attributable to common stockholders - basic ( 2.27 1.63 ( 1.59 3.24

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Income (Loss) income attributable to common stockholders - diluted

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Net income (loss) income per share applicable to common stockholders - diluted ( 2.27 1.63 ( 1.59 3.24

Anti-dilutive potential common stock equivalents excluded from the calculation of net income (loss) income per share:

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The Company's potential dilutive securities include stock options, unvested restricted common stock and shares issuable under the 2015 Employee Stock Purchase Plan. The effect of dilutive securities was calculated using the treasury stock method. The anti-dilutive potential common stock equivalents for the six months ended June 30, 2026 and the three and nine months ended June September 30, 2025 2026 were excluded from the computation of diluted net loss per share attributable to common stockholders because those stock options to purchase common stock, restricted stock units, and shares issuable under employee stock purchase plan had an anti-dilutive impact as the Company reported a net loss from continuing operations attributable to common stockholders for those periods. The anti-dilutive potential common stock equivalents for the three months ended June 30, 2026 and the six nine months ended June September 30, 2025 were excluded from the computation of diluted net income per share attributable to common stockholders because those stock options to purchase common stock and restricted stock units had an anti-dilutive impact due to the assumed proceeds per share using the treasury stock method being greater than the average fair value of the Company's common shares for those periods. For that same period, the warrants to purchase common stock were excluded because the exercise price of the Tranche A warrants is greater than the average fair value of the Company' s common shares.

Note 9: Commitments and Contingencies 2 changes

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In the ordinary course of business, the Company may provide indemnification of varying scope and terms to vendors, lessors, business partners and other parties with respect to certain matters including, but not limited to, losses arising out of breach of such agreements or from intellectual property infringement claims made by third parties. In addition, the Company has entered into indemnification agreements with members of its board of directors and its officers that will require the Company, among other things, to indemnify them against certain liabilities that may arise by reason of their status or service as directors or officers. The maximum potential amount of future payments the Company could be required to make under these indemnification agreements is, in many cases, unlimited. To date, the Company has not incurred any material costs as a result of such indemnifications. The Company does not believe that the outcome of any claims under indemnification arrangements will have a material effect on its financial position, results of operations or cash flows, and it has no t accrued any liabilities related to such obligations in its condensed consolidated financial statements as of June September 30, 2026 or December 31, 2025.

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The Company did no t accrue any liabilities related to legal contingencies in its condensed consolidated financial statements as of June September 30, 2026 or December 31, 2025 .

Note 10: Income taxes 1 change

The Company did no t provide for any income taxes in its condensed consolidated state ment of operations and comprehensive (loss) income for the three and six nine months ended June September 30, 2026 and 2025. Management considered the Company's history of cumulative net losses incurred since inception, its early stage of development of its product candidates, and its projection of book and tax losses for the year ending December 31, 2026. Based on its evaluation of the positive and negative evidence bearing upon its ability to realize its deferred tax assets, the Company determined that it is more likely than not that it will not realize such benefits. Accordingly, the Company has recorded a full valuation allowance against its deferred tax assets as of June September 30, 2026 and December 31, 2025, and has not recorded any income taxes for the three and six nine months ended June September 30, 2026 and 2025 . Management reevaluates the positive and negative evidence at each reporting period.

Note 11: Related Party Transactions 4 changes

1 unchanged paragraph

As of June September 30, 2026 and December 31, 2025, there was $ 3,278 included in Accrued Liabilities due to SPN - related party on the Company's condensed consolidated balance sheet, which represents amounts due to SPN pursuant to the Purchase Agreement, which are further described in Note 3, Discontinued Operations and TSA . During the three and six nine months ended June September 30, 2026 and 2025, the Company paid $ 0 , $ 0 , $ 0 6,284 , and $ 1,309 7,593 , to SPN related to the Purchase Agreement during the periods, respectively.

As described in Note 3, Discontinued Operations and TSA , the Company entered into the TSA with NESA, an affiliate of SPN, in connection with the Transaction, through which the Company provided certain manufacturing services until December 31, 2025, and other transition services, for the duration specified in the schedule to the TSA for each service. For the three and six nine months ended June September 30, 2026 and 2025, the Company recognized $ 0 , $ 0 , $ 3,490 2,028 , and $ 9,799 11,827 , respectively, of TSA reimbursement income in other income in the Company's condensed consolidated statements of operations and comprehensive (loss) income. For the three and six nine months ended June September 30, 2026 and 2025, the Company incurred $ 0 , $ 0 , $ 1,689 736 , and $ 5,216 5,952 , respectively, of expenses related to manufacturing services and $ 0 , $ 0 , $ 1,261 76 , and $ 3,509 3,585 , respectively, of TSA labor and passthrough expenses to support the transition services, including finance and accounting, information technology, human resources, operations, and other services.

During the three and six nine months ended June September 30, 2026 and 2025, $ 0 , $ 0 , $ 4,585 1,756 , and $ 11,987 13,743 , respectively, was billed to NESA related to transition services performed by the Company, and the Company received payments of $ 0 , $ 360 , $ 6,464 2,212 , and $ 63,173 90,385 , respectively, related to transition services performed. The payments received in the first quarter of 2025 included the installment payment of $ 50,000 received in January 2025 and the second installment payment of $ 25,000 received in July 2025 which was were conditioned on the Company's material compliance with obligations under the TSA. As of June September 30, 2026 and December 31, 2025, the Company had $ 25,000 12,500 and $ 360 , respectively, in accounts receivable due from SPN - related party, respectively, and $ 0 and $ 169 included in prepaid expenses and other current assets, respectively, relating to the sale of certain manufacturing and laboratory equipment, in the Company's condensed consolidated balance sheets.

As described in Note 3, Discontinued Operations and TSA, the Company entered into the APA Amendment with SPN pursuant to which the parties agreed to terminate SPN's obligations with respect to the Second Sales Milestone Payment, the Third Sales Milestone Payment and eliminate the Milestone Interest Payments that would have been due from the Company to SPN. In consideration for the foregoing, SPN agreed to a one-time payment to the Company of $ 25,000 (the "Milestone Termination Payment"), which amount is payable in two equal installments of $ 12,500 , with the first installment paid on July 1, 2026 and the second installment payable paid on October 1, 2026. Th e Company recorded a Gain on Sale of VOWST Business within continuing operations and the corresponding receivable due from SPN of $ 25,000 during the three months ended June 30, 2026 because the amount is fixed consideration and is no longer contingent on a future event.

Note 12: Segment Reporting 8 changes

Six Months Ended June 30,

The Company manages its operations as a single operating segment for the purposes of assessing performance and making operating decisions, and thus reports as a single reportable segment. The Company ' s interim Chief Executive Officer , who is the Chief Operating Decision Maker ("CODM " ), manages and allocates resources to the operations of the Company on a total company basis by assessing the overall level of resources available and how to best deploy these resources across functions and research and development projects that are in line with the Company' s long-term company-wide strategic goals. The CODM uses the Company's consolidated net income (loss) to monitor actual results versus the budget in assessing segment performance and the allocation of resources. The measure of segment assets is reported on the consolidated balance sheets as total assets.

The Company's reportable segment net income (loss) income for the three and six nine months ended June September 30, 2026 and 2025, consisted of the following (in thousands):

Three Months Ended June September 30,

Nine Months Ended September 30,

7 unchanged paragraphs

Other segment (income) expense (1)

Other segment expense (income) (1)

[1] Other segment (income) expense (income) includes manufacturing services expenses, research and development expenses on early stage programs, interest income and other (income) expense (income), net.

1 unchanged paragraph

Note 13: Subsequent Events 7 changes

On July 31, 2026, the Company entered into the Third Amendment to Lease and Termination Agreement (the "Termination

Agreement") with BMR-Sidney Research Campus LLC, a Delaware limited liability company (the "Sidney Street Landlord"), the purpose of which is to effect an early termination of one of the Company's leases, reducing the Company's ongoing annual facilities costs and materially eliminating the Company's primary restoration obligations. The Termination Agreement amends the Lease Agreement, dated November 11, 2015, by and between the Company and the Sidney Street Landlord (as amended, the "Lease"), pursuant to which the Company leases approximately 68,636 rentable square feet of office, laboratory, and manufacturing space located at 200 Sidney Street, Cambridge, Massachusetts (the "Existing Premises").

The Termination Agreement provides for (i) effective as of August 1, 2026, the surrender by the Company to the Landlord of an aggregate area of approximately 21,295 rentable square feet of the Existing Premises (the "Early Termination Premises"), and (ii) the revision of the expiration date of the term of the Lease with respect to the remaining 47,341 rentable square feet of the Existing Premises (the "Renewal Premises") from January 13, 2031 to December 31, 2026. As consideration for the Termination Agreement, the Company agreed to pay the following: (i) an increase to the letter of credit held by the Sidney Street Landlord in the amount of $ 2,238 (for a total letter of credit of $ 3,638 ), which the Sidney Street Landlord will apply toward rent and operating expenses through the term of the Renewal Premises, (ii) on or before January 4, 2027, a deferred payment of $ 3,850 representing an additional termination payment, and (iii) an equity issuance of shares of the Company's common stock valued at $ 500 (the "Share Issuance").

On July 31, 2026, in connection with the Share Issuance, the Company entered into a Stock Issuance Agreement (the "Stock Issuance Agreement") with the Sidney Street Landlord, pursuant to which the Company agreed to issue 103,520 shares of Common Stock (the "Shares") to the Sidney Street Landlord at a price per share of $ 4.83 , which was the closing price of the Common Stock on the Nasdaq Global Select Market ("Nasdaq") on July 31, 2026.

The Shares are to be issued no later than five business days after the Execution Date. The Shares are registered under the Company's registration statement on Form S-3 (File No. 333-273794), pursuant to a prospectus supplement, dated July 31, 2026 and the accompanying base prospectus, dated August 15, 2023, and will be freely tradeable when issued. In the event the value of the Shares on the date of issuance (calculated using the closing price on Nasdaq on such date) (the "Issuance Date Value") is less than $ 500 , the Company is obligated to pay to the Sidney Street Landlord, in cash, the difference between $ 500 , and the Issuance Date Value. The Stock Issuance Agreement contains customary representations and warranties of the Company and the Sidney Street Landlord and customary conditions to the share issuance.

On October 22, 2026 , Marella Thorell resigned from her positions as Executive Vice President and Chief Financial Officer of the Company, effective November 15, 2026 (the "Effective Date"). In connection with her resignation, on November 2, 2026, the Company and Ms. Thorell entered into an advisory agreement (the "Advisory Agreement") under which Ms. Thorell will provide advisory services to the Company from her resignation through March 31, 2027.

As of the Effective Date, the Board has designated Richard Kender, the Company's Executive Chair and Interim Chief Executive Officer and principal executive officer, to serve as the Company's principal financial officer and principal accounting officer. Mr. Kender will continue to serve as the Company's Executive Chair and Interim Chief Executive Officer and principal executive officer, and he will not receive any additional compensation for his role as interim principal financial officer and principal accounting officer during this period.

MD&A 68 changes

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In the context of preventing infection for patients undergoing allo-HCT for hematological cancer, we believe that our technology may be replicable across different bacterial pathogens with the potential to develop live biotherapeutics to protect a range of medically compromised patients at risk of antimicrobial resistance, or AMR, infections and bloodstream infections, or BSIs, that can result from a compromised mucosal epithelial barrier and that can be a major cause of mortality. SER-155, our most advanced LBP candidate, is an investigational, oral, live biotherapeutic designed to decolonize GI pathogens, improve GI mucosal epithelial barrier integrity, and induce immune homeostasis to prevent bacterial BSIs, including those that can harbor antimicrobial resistance, as well as other pathogen-associated negative clinical outcomes in patients undergoing allogeneic hematopoietic stem cell transplantation, or allo-HCT. In our placebo-controlled Phase 1b study of SER-155 in allo-HCT, SER-155 was associated with a 77% relative risk reduction in bacterial BSIs and a significant reduction in systemic antibiotic exposure as well as a lower incidence of febrile neutropenia, as compared to placebo through day 100 post HCT. SER-155 was generally well tolerated, with no observed treatment-related serious adverse events. Drug pharmacology data demonstrated that SER-155 significantly promoted mucosal epithelial barrier integrity and had a positive impact on biomarkers of systemic inflammation and immune homeostasis. In November 2026, we announced that results from our Phase 1b clinical study of SER-155 in adults undergoing allo-HCT were published in Nature Medicine , marking an important milestone for the program and providing recognition of the innovative science being advanced by the Company. Following advancement of key startup activities for the SER-155 Phase 2 study in allo-HCT, including the submission of a final protocol to the FDA in January 2026, study site evaluation and qualification with our CRO, and manufacturing of drug substance, we have paused additional investment in that program while continuing to seek funding for the Phase 2 study.

We are also evaluating SER-155 in 15 patients with moderate-to-severe (Grade 2- 3) immune checkpoint-related enterocolitis, or is also being developed to prevent irEC, through an investigator-sponsored trial, or IST, with Memorial Sloan Kettering the aim of enabling patients to continue their ICI cancer Center, an institution with whom we have had a decade-long collaboration. therapy and avoid the need for immunosuppressive steroid treatment. irEC is among the most frequent and severe immune-related adverse events, or irAEs, in recipients of immune checkpoint inhibitor, or ICI, therapy and can be observed in up to 50% of ICI patients, with rates varying based on cancer drug and treatment regimen. Moderate to severe (Grade 2-3) irEC occurs in approximately 25% of all ICI therapy recipients in the United States. Clinical guidelines for patients who experience Grade 2 or higher irEC require treatment, which could include halting of ICI cancer treatment and initiating a systemic immunosuppressive corticosteroid, a biologic, or both, which are associated with clinically meaningful toxicities and other negative consequences, including systemic immunosuppression, increased infection risk, metabolic complications, and potential interference with cancer treatment. In 2026, it is estimated that there are a total of 300,000 patients on ICI cancer therapy in the United States, and this figure is increasing as ICI are being used across multiple cancer types, in earlier intervention and in combination with other cancer therapies. ICIs can cause a wide range of irAEs with links to T cell biology and epithelial barrier inflammation, both of which are biological functions shown in our preclinical and clinical pharmacology data to be positively impacted by SER-155. Top-line data from the IST showed that 80% of SER-155 recipients achieved an immunosuppressive-free clinical response of diarrhea (a primary symptom of irEC) at Day 15, which met the primary endpoint, defined as at least a 1-grade improvement in diarrhea symptoms without immunosuppressive therapy (i.e., a corticosteroid and/or biologic immunomodulator drug). All 12 participants with immunosuppressive-free clinical response at Day 15 had the same, or better, diarrhea grade at Day 43. SER-155 was well tolerated, with no drug-related serious adverse events observed. In addition, drug pharmacology translational biomarkers of gastrointestinal inflammation (fecal calprotectin) and of mucosal epithelial barrier integrity (fecal albumin), which were elevated in participants at baseline, prior to SER-155 dosing, both showed decreases post SER-155 treatment, with statistically significant reductions achieved by Day 43, reflecting improvement in two mechanistically linked dimensions of irEC disease. Participants in the study were on a wide range of ICI types, including PD-1 inhibitors (Keytruda®, Opdivo®, Zynyz®), PD-L1 inhibitors (Imfinzi®, Bavencio®), CTLA-4 inhibitors (Yervoy®, Imjudo®), LAG-3 inhibitor (Opdualag®), and combinations thereof. Clinical guidelines for these patients stipulate halting of ICI cancer treatment and initiation of systemic immunosuppressive corticosteroids, which are associated with clinically meaningful toxicities and other negative consequences, including: systemic immunosuppression, increased infection risk, metabolic complications, and potential interference with cancer treatment.

SER-155 was evaluated in 15 patients with moderate-to-severe irEC through an investigator-sponsored trial, or IST, with Memorial Sloan Kettering Cancer Center, an institution with whom we have had a decade-long collaboration. Top-line data from the IST showed that 80% of SER-155 recipients achieved an immunosuppressive-free clinical response of diarrhea (a primary symptom of irEC) at Day 15, which met the primary endpoint, defined as at least a 1-grade improvement in diarrhea symptoms without immunosuppressive therapy (i.e., a corticosteroid and/or biologic immunomodulator drug). All 12 participants with immunosuppressive-free clinical response at Day 15 had the same, or better, diarrhea grade at Day 43. SER-155 was well tolerated, with no drug-related serious adverse events observed. In addition, drug pharmacology translational biomarkers of gastrointestinal inflammation (fecal calprotectin) and of mucosal epithelial barrier integrity (fecal albumin), which were elevated in participants at baseline, prior to SER-155 dosing, both showed decreases post SER-155 treatment, with statistically significant reductions achieved by Day 43, reflecting improvement in two mechanistically linked dimensions of irEC disease. Participants in the study were on a wide range of ICI types, including PD-1 inhibitors (Keytruda®, Opdivo®, Zynyz®), PD-L1 inhibitors (Imfinzi®, Bavencio®), CTLA-4 inhibitors (Yervoy®, Imjudo®), LAG-3 inhibitor (Opdualag®), and combinations thereof.

We believe there remains a significant unmet need for systemic immunosuppressive-free therapeutic approaches that reduce gastrointestinal inflammation and diarrhea and may allow patients who develop irEC to avoid disruption of their life-saving ICI cancer treatment and the promising study results support continued development of SER-155 to treat irEC. We are engaging with potential partners, including companies with ICI franchises, as we evaluate next steps for the development of SER-155 in irEC patients.

We plan to advance SER-155 into a next study in irEC evaluating prophylactic oral administration and dose optimization in patients initiating ICI cancer therapy. The prevention approach aims to reduce incidence of irEC, keep patients on ICI therapy and avoid immunosuppressive irEC treatments. The study is being designed in consultation with leading oncologists and irEC experts. As part of the development program, we intend to engage with FDA to discuss study design and potential endpoints including incidence of irEC (all grades), ICI administration and safety. In addition, we intend to seek feedback from FDA on our plans to optimize dosing for prophylaxis, which could include a healthy volunteer study component or a dosing lead-in element, the results of which are expected to inform the prophylactic dosing course before initiating dosing in the broader ICI population. We expect to submit the study design to the FDA in the first quarter of 2027. We are also engaging with potential partners as we evaluate next steps for the development of SER-155 in irEC.

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We have built and deploy deployed a reverse translational platform and knowledge base for the discovery and development of live biotherapeutics and maintain extensive proprietary know-how that may be used to support future research and development efforts. This platform incorporates high-resolution analysis of human clinical data to identify microbiome biomarkers associated with disease and non-disease states; preclinical screening using human cell-based assays and in vitro/ex vivo and in vivo disease models customized for live biotherapeutics; and a strain library and associated microbiological capabilities that span broad biological and functional breadth. This platform and knowledge base are integrated through a proprietary knowledge graph and agentic artificial intelligence, enabling rapid identification of specific microbes, microbial genes, and microbial metabolites/peptides associated with disease and the design of therapeutic consortia of bacteria for specific pharmacological properties to restructure the gut microbiome and modulate functional pathways associated with disease. In addition, we own a valuable intellectual property estate related to the development and manufacture of live biotherapeutics.

1 unchanged paragraph

Our product candidates are in early-stage clinical or preclinical development. Our ability to achieve profitability will depend heavily on the successful development and eventual commercialization of one or more of our product candidates. Since our inception, we have incurred significant operating losses. Our net loss from operations was $42.2 $58.3 million for the six nine months ended June September 30, 2026. As of June September 30, 2026, we had an accumulated deficit of $987.7 $1,004.0 million.

3 unchanged paragraphs

As of June September 30, 2026, we had cash and cash equivalents totaling $15.6 $17.5 million. Based on our currently available cash resources, the $12.5 million Milestone Termination Payment received from SPN in July 2026, and the expected receipt of the additional $12.5 million Milestone Termination Payment due from SPN in October 2026, and considering our future operating plans, we anticipate that we will require additional funding following the first quarter of 2027. In accordance with applicable accounting standards, we evaluated whether there are conditions and events, considered in the aggregate, that raise substantial doubt about our ability to continue as a going concern within 12 months after the date of the issuance of the condensed consolidated financial statements included elsewhere in this Quarterly Report on Form 10-Q. In performing this analysis, we excluded certain elements of our operating plan that cannot be considered probable of occurring. Under the applicable accounting standards, any future transactions or equity issuances cannot be considered probable, as these events are outside our control. Accordingly, management has concluded that substantial doubt exists about our ability to continue as a going concern for 12 months from the date the condensed consolidated financial statements included elsewhere in this Quarterly Report on Form 10-Q are issued. See "Risk Factors— Risks Related to Our Financial Position and Need for Additional Capital — We have identified conditions and events that raise substantial doubt regarding our ability to continue as a going concern ."

1 unchanged paragraph

On February 12, 2026, we announced that we implemented cost reduction actions, including reducing our workforce by approximately 30%. During the three nine months ended March 31, September 30, 2026, we incurred approximately $0.9 million in restructuring costs, primarily related to severance costs, of which $0.7 million was paid during the first quarter of 2026, and the remainder was paid out in the second quarter of 2026.

On June 5 and August 5, 2026, we announced that we amended our lease with two of our landlord leases for a portion of our space in Cambridge, Massachusetts, under which we reduced our leased space, rental rate and facility operating expenses. The restructured lease is leases are expected to materially reduce the Company's our ongoing annual facility-related costs and long-term obligations. On July 31, 2026, we entered into the Third Amendment to Lease and Termination Agreement with BMR-Sidney Research Campus LLC, the purpose of which was to effect an early termination of our lease at 200 Sidney Street, Cambridge, Massachusetts, reducing our ongoing annual facilities costs and materially eliminating our primary restoration obligations. Refer to Note 6, Leases , included elsewhere in this Quarterly Report on Form 10-Q for more information.

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We have been collaborating with Memorial Sloan Kettering Cancer Center for over a decade on the impact of the GI microbiome on immune related diseases and cancer; recently this long-standing collaboration included a single arm, open-label Phase 1b investigator-sponsored trial, or IST, evaluating SER-155 as a potential first-line treatment in irEC. The study evaluated SER-155 in 15 patients with moderate-to-severe (Grade 2- 3) irEC, irEC is among the most frequent and severe immune-related adverse events, or irAEs, in recipients of immune checkpoint inhibitor, or ICI, therapy and can be observed in up to 50% of patients, with rates varying based on cancer drug and treatment regimen. Moderate to severe irEC occurs in which affects approximately 25% of all ICI therapy recipients in the United States. ICIs can cause a wide range of irAEs with links to T cell biology and epithelial barrier inflammation, both of which are biological functions shown in our preclinical and Clinical pharmacology data to be positively impacted by SER-155. Participants in the study were on a wide range of ICI types, including PD-1 inhibitors (Keytruda®, Opdivo®, Zynyz®), PD-L1 inhibitors (Imfinzi®, Bavencio®), CTLA-4 inhibitors (Yervoy®, Imjudo®), LAG-3 inhibitor (Opdualag®), and combinations thereof. Clinical guidelines for these patients who experience Grade 2-3 2 or higher irEC stipulate require treatment, which could include halting of ICI cancer treatment and initiation of systemic immunosuppressive corticosteroids, which are associated with clinically meaningful toxicities and other negative consequences, including systemic immunosuppression, increased infection risk, metabolic complications, and potential interference with cancer treatment. There remains a significant unmet need for systemic, immunosuppressive-free therapeutic approaches that reduce gastrointestinal inflammation and diarrhea and may allow patients who develop irEC to avoid disruption of their life-saving ICI cancer treatment. SER-155 is designed to promote mucosal healing and modulate inflammation without systemic immunosuppressives, which carry Participants in the risk study were on a wide range of toxicity and ICI efficacy impact. types, including PD-1 inhibitors (Keytruda®, Opdivo®, Zynyz®), PD-L1 inhibitors (Imfinzi®, Bavencio®), CTLA-4 inhibitors (Yervoy®, Imjudo®), LAG-3 inhibitor (Opdualag®), and combinations thereof.

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We believe data from this IST further supports the potential for live biotherapeutics to address a significant unmet need among the large population of cancer patients receiving ICIs for systemic immunosuppressive-free therapeutic approaches that have the potential to reduce gastrointestinal inflammation and diarrhea and may allow patients who develop irEC to avoid disruption of their life-saving ICI cancer treatment.

We believe data from this IST further supports the potential for live biotherapeutics to address a significant unmet need among the large population of cancer patients receiving ICIs for systemic immunosuppressive-free therapeutic approaches that have the potential to reduce gastrointestinal inflammation and diarrhea and may allow patients who develop irEC to avoid disruption of their life-saving ICI cancer treatment. We plan to advance SER-155 into a next study in irEC evaluating prophylactic oral administration and dose optimization in patients initiating ICI cancer therapy. The prevention approach aims to reduce incidence of irEC, keep patients on ICI therapy and avoid immunosuppressive irEC treatments. The study is being designed in consultation with leading oncologists and irEC experts. As part of the development program, we intend to engage with FDA to discuss study design and potential endpoints including incidence of irEC (all grades), ICI administration, and safety. In addition, we intend to seek feedback from FDA on our plans to optimize dosing for prophylaxis, which could include a healthy volunteer study component or a dosing lead-in element, the results of which are expected to inform the prophylactic dosing course before initiating dosing in the broader ICI population. We expect to submit the study design to the FDA in the first quarter of 2027.

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In November 2026, we announced that results from our Phase 1b clinical study of SER-155 in adults undergoing allo-HCT were published in Nature Medicine , marking an important milestone for the program and providing recognition of the innovative science being advanced by the Company.

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In October 2026, we presented preclinical data for SER-428 at IDWeek in Washington, D.C. in a poster presentation. In preclinical in vivo pathogen colonization models, oral administration of SER-428 led to up to a 1,000- fold reduction of Enterococcus faecium and Klebsiella pneumoniae. These findings support the potential of SER-428 to reduce GI pathogen abundance, bloodstream infections, and antimicrobial resistance in high-risk patients. The SER-428 liquid formulation was stable, including during the intended in-use period and following simulated feeding tube delivery.The findings presented provided important preclinical support for both the biological activity and the formulation characteristics of SER-428 as we continue to advance our clinical development plans for SER-428.

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We have an extensive patent portfolio directed to rationally designed ecologies of spores and microbes. The portfolio includes both company-owned patents and applications, and those that we have rights to as licensee. The patents and applications included in our portfolio cover both composition of matter and methods (e.g., method of treating). Our intellectual property rights related to SER-155, SER-147, SER-428 and/or SER-603 extend through at least part of 2046 (not including any potential term extension). We plan on continuing to broaden our patent portfolio. Currently, we have 20 21 active patent families, which includes 16 nationalized applications, three at the PCT stage, and one at the provisional stage. To date, we have obtained issuance of 33 U.S. patents (which includes three as licensee). Of the issued U.S. patents, 13 U.S. patents (including one as licensee) have been assigned to Nestlé Health Science as part of its purchase of VOWST.

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We recognized an impairment loss of $5.8 $5.7 million and $11.5 million for the three and nine months ended September 30, 2026, respectively, related to one of our operating leases and the associated leasehold improvements as an indicator of impairment was deemed to exist and the carrying value of the asset group exceeded its fair value. Refer to Note 6, Leases , included elsewhere in this Quarterly Report on Form 10-Q, for more information.

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Since our inception in 2010, we have not recorded any U.S. federal or state income tax benefits for the net losses we have incurred in each year or our earned research and development tax credits, due to our uncertainty of realizing a benefit from those items. We did not provide for any income taxes in the three and six nine months ended June September 30, 2026 or 2025.

1 unchanged paragraph

Our condensed consolidated financial statements are prepared in accordance with accounting principles generally accepted in the United States. The preparation of these condensed consolidated financial statements requires the application of appropriate technical accounting rules and guidance, as well as the use of estimates. The application of these policies necessarily involves judgments regarding future events. These estimates and judgments, in and of themselves, could materially impact the condensed consolidated financial statements and disclosures based on varying assumptions. The accounting policies discussed in our Annual Report on Form 10-K for the fiscal year ended December 31, 2025, filed with the SEC on March 12, 2026, or the Annual Report, are considered by management to be the most important to an understanding of the consolidated financial statements because of their significance to the portrayal of our financial condition and results of operations. There have been no material changes to that information disclosed in our Annual Report during the three and six nine months ended June September 30, 2026.

1 unchanged paragraph

Comparison of Three Months Ended June September 30, 2026 and 2025

The following table summarizes our results of operations for the three months ended June September 30, 2026 and 2025:

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Total revenue was $0.7 $0.6 million and $0.4 million for the three months ended June September 30, 2026 and 2025 consisting of costs reimbursable under the CARB-X grant that we were awarded in the third quarter of 2025.

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Research and development expenses were $9.1 $5.6 million for the three months ended June September 30, 2026 and $12.9 $12.6 million for the three months ended June September 30, 2025. The decrease of $3.8 $7.0 million was primarily due to the following:

• a decrease in personnel-related costs of $2.4 $2.5 million primarily due to a decrease in salaries, bonuses, employee benefits expenses, and stock-based compensation expense due to lower headcount;

• a decrease of $0.9 $3.9 million in expenses related to our live biotherapeutics platform and research and development operations primarily due to the conclusion of the TSA with Nestlé as of December 31, 2025, under which we incurred costs related to inventory shipments and storage of sample material that were reimbursed by Nestlé in 2025, as well as a reduction in facilities costs related to the Lease Amendment that was effective during the second quarter of 2026 and the Termination Agreement that was effective during the third quarter of 2026;

1 unchanged paragraph

• an increase of $0.5 $0.3 million in expenses related to our early stage programs primarily related to activities associated with the development of SER-428, for which the related reimbursements received from CARB-X are included within grant revenue.

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General and administrative expenses were $7.1 $5.4 million for the three months ended June September 30, 2026 compared to $10.3 $9.5 million for the three months ended June September 30, 2025. The decrease of $3.2 $4.0 million was primarily due to the following:

• a decrease in personnel-related costs of $1.1 $1.9 million primarily due to a decrease in salaries, bonuses, employee benefits expenses, and stock-based compensation expenses due to lower headcount;

• a decrease in professional fees of $0.7 $0.3 million due to lower legal and consulting fees, and

• a decrease in facility-related and other costs of $1.4 $1.8 million primarily related to information technology costs that were incurred as part of the TSA and reimbursed by Nestlé that did not reoccur in 2026, as well as a reduction in facilities costs related to the Lease Amendment that was effective during the second quarter of 2026 and the Termination Agreement that was effective during the third quarter of 2026.

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Impairment of long-lived assets was $5.8 $5.7 million for the three months ended June September 30, 2026, consisting of $5.6 $3.7 million related to right-of-use assets and $0.2 $2.0 million related to the associated leasehold improvements. Refer to Note 6, Leases , included elsewhere in this Quarterly Report on Form 10-Q, for more information.

1 unchanged paragraph

Other income (expense), net was $25.9 $0.1 million of income expense and $5.0 $30.7 million of income for the three months ended June September 30, 2026 and 2025, respectively. The increase decrease of $20.8 $30.8 million in other income, net was primarily due to the $25.0 $27.2 million gain on sale of the VOWST business related primarily due to the Milestone Termination installment payment expected to be received from SPN Nestlé in July and October 2026, partially offset by $3.5 2025 that was conditioned on our material compliance with obligations under the TSA, as well as $2.0 million of reimbursement income associated with the performance of TSA services in the second third quarter of 2025 that did not reoccur in 2026 due to the conclusion of the TSA at the end of 2025, a decrease in sublease income of $1.1 million, and a decrease in interest income of $0.3 $0.4 million due to our lower cash balance.

Comparison of Six Nine Months Ended June September 30, 2026 and 2025

The following table summarizes our results of operations for the six nine months ended June September 30, 2026 and 2025:

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Total revenue was $1.1 million for the six months ended June 30, 2026 consisting of costs reimbursable under the CARB-X grant that we were awarded in the third quarter of 2025.

Total revenue was $1.7 million and $0.4 million for the nine months ended September 30, 2026 and 2025 consisting of costs reimbursable under the CARB-X grant. The increase reflects a full nine months of activity in 2026, compared with one quarter in 2025, as the award became effective in the third quarter of 2025.

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Research and development expenses were $22.3 $27.9 million for the six nine months ended June September 30, 2026 and $24.8 $37.4 million for the six nine months ended June September 30, 2025. The decrease of $2.4 $9.4 million was primarily due to the following:

• a decrease in personnel-related costs of $2.6 $5.1 million primarily due to a decrease in salaries, bonus, employee benefits expenses, and stock-based compensation expense due to lower headcount; and

• a decrease of $0.9 $1.8 million in expenses related to our SER-155 program as investment related to preparation for the Phase 2 study of SER-155 in allo-HCT was paused in February 2026; partially offset by and

• an increase a decrease of $0.5 $3.4 million in expenses related to our live biotherapeutics platform and research and development operations primarily due to a reduction in facilities costs related to the Lease Amendment that was effective during the second quarter of 2026 and the Termination Agreement that was effective during the third quarter of 2026, partially offset by an increase in facilities costs due to the conclusion of the TSA with Nestlé as of December 31, 2025, under which a portion of our facility costs and other operational costs were included within manufacturing services expenses and were reimbursed by Nestlé in 2025; partially offset by a reduction in facilities costs related to the Lease Amendment that was effective during the second quarter of 2026; and

• an increase of $0.6 $0.9 million in expenses related to our early stage programs primarily related to activities associated with the development of SER-428, for which the related reimbursements received from CARB-X are included within grant revenue.

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General and administrative expenses were $15.1 $20.6 million for the six nine months ended June September 30, 2026 compared to $22.1 $31.6 million for the six nine months ended June September 30, 2025. The decrease of $7.0 $11.0 million was primarily due to the following:

• a decrease in personnel-related costs of $1.9 $3.8 million primarily due to a decrease in salaries, bonus, employee benefits expenses, and stock-based compensation expenses due to the decrease in headcount period over period; and

• a decrease in professional fees of $2.2 $2.5 million due to lower legal and consulting fees, and

• a decrease in facility-related and other costs of $2.9 $4.7 million primarily related to information technology costs that were incurred as part of the TSA and reimbursed by Nestlé that did not reoccur in 2026 and facilities costs associated with the Termination Agreement that was effective during the third quarter of 2026.

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Impairment of long-lived assets was $5.8 $11.5 million for the six nine months ended June September 30, 2026, consisting of $5.6 $9.2 million related to right-of-use assets and $0.2 $2.3 million related to the associated leasehold improvements. Refer to Note 6, Leases , included elsewhere in this Quarterly Report on Form 10-Q, for more information.

Other income (expense), net was $26.8 $26.7 million of income and $64.9 $95.6 million of income for the six nine months ended June September 30, 2026 and 2025, respectively. The decrease of $38.1 $68.9 million in other income, net was due to the lower gain on sale of VOWST business in 2026 compared to 2025 of $27.4 $54.6 million, as well as $9.8 $11.8 million of reimbursement income associated with the performance of TSA services in the first half of 2025 that did not reoccur due to the conclusion of the TSA at the end of 2025, and a decrease in interest income of $0.7 $1.1 million due to our lower cash balance. The $52.4 $79.6 million gain on sale of the VOWST business recognized in the six nine months ended June September 30, 2025 was primarily due to the $50.0 $75.0 million installment payment payments received from Nestlé in January and July 2025 that was were conditioned on our material compliance with obligations under the TSA, while the $25.0 million gain on sale of VOWST business recognized in the six nine months ended June September 30, 2026 was related to the Milestone Termination Payment expected to be received from SPN in July and October 2026.

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(iii) prepayment of the $60 million Prepaid Milestone tied to the achievement of the First Sales Milestone of worldwide annual net sales of the Product of $150 million, which was paid in cash at Closing, which Prepaid Milestone will accrue interest at a fixed rate of 10% per annum until the First Sales Milestone is achieved and 5% per annum thereafter until the earlier of (x) the date on which the Prepaid Milestone, plus accrued interest thereon, has been repaid in full by set-off and (y) the last day of the Milestone Period; and (iv) future Milestone Payments of (x) $125 million tied to the achievement of worldwide annual net sales of the Product of $400 million and (y) $150 million tied to the achievement of worldwide annual net sales of the Product of $750 million, during the Milestone Period from Closing until December 31 of the calendar year in which the tenth anniversary of Closing occurs.

As they are earned, the Milestone Payments will be satisfied as follows: (i) first, by set-off against all accrued interest on the Prepaid Milestone until the amount of such accrued interest has been paid in full, (ii) second, by set-off against the outstanding balance of the Prepaid Milestone until the Prepaid Milestone has been repaid in full and (iii) thereafter, in cash. If any amount of the Prepaid Milestone (and any accrued interest thereon) remains outstanding as of following the last day of the Milestone Period (defined below), the balance thereof (together with any interest accrued thereon) will be forgiven and the right of set-off of SPN with respect thereto will be deemed forfeited. The installment payment received on July 1, 2025 was offset by $1.4 million that we paid to Nestlé on the same date related to certain employment obligations assumed by SPN through the period prior to the Closing Date.

(iii) prepayment of the $60 million Prepaid Milestone, which was paid in cash at Closing; and (iv) potential future sales milestone payments of $125.0 million and $150.0 million, tied to the achievement of worldwide annual net sales of the Product of $400.0 million and $750.0 million, respectively, during the Milestone Period.

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On June 2, 2026, we entered into Amendment No. 1 (the "APA Amendment") to the Purchase Agreement. Pursuant to the APA Amendment, the parties agreed to terminate SPN's obligations with respect to the Milestone Payments described in clause (iv) above and eliminate the milestone interest payments that would have been due from us to SPN. In consideration for the foregoing, SPN agreed to a one-time payment to us of $25.0 million (the "Milestone Termination Payment"), which amount is payable in two equal installments of $12.5 million, with the first installment paid on July 1, 2026 and the second installment payable paid on October 1, 2026. As a result, no further milestone payments are payable to us under the Purchase Agreement.

In May 2021, we entered into a Sales Agreement, or the Sales Agreement, with Cowen and Company, LLC, or Cowen, to sell shares of our common stock, with aggregate gross sales proceeds of up to $150.0 million, from time to time, through an "at the market" equity offering program under which Cowen acts as sales agent. During the three and six nine months ended June September 30, 2026, we sold 232,286 113,643 and 258,082 371,725 shares of common stock under the Sales Agreement, at an average price of approximately $7.99 $7.45 and $8.78 $8.37 per share, raising aggregate net proceeds of approximately $1.7 $0.7 million and $2.1 $2.9 million after deducting an aggregate commission of approximately 3%. During the six nine months ended June September 30, 2025, we sold 54,806 shares of common stock under the Sales Agreement, at an average price of approximately $18.60 per share, raising aggregate net proceeds of approximately $1.0 million after deducting an aggregate commission of approximately 3% and other issuance costs. As of June September 30, 2026, we have sold 2,446,777 2,560,420 shares of common stock under the Sales Agreement, at an average price of approximately $26.16 $25.33 per share, raising aggregate net proceeds of approximately $61.4 $62.2 million after deducting an aggregate commission of approximately 3% and other issuance costs. On August 13, 2026, we entered into a Sales Agreement, or the 2026 Sales Agreement, with TD Securities (USA) LLC, or TD Cowen, to sell shares of our common stock, with aggregate gross sales proceeds of up to $100.0 million, from time to time, through an "at the market" equity offering program under which TD Cowen acts as sales agent. During the three months ended September 30, 2026, no shares were sold under the 2026 Sales Agreement.

As of June September 30, 2026, we had cash and cash equivalents totaling $15.6 $17.5 million and an accumulated deficit of $987.7 $1,004.0 million. For the six nine months ended June September 30, 2026, we incurred a loss from operations of $42.2 $58.3 million . We expect that our operating losses and negative cash flows will continue for the foreseeable future.

Under applicable accounting standards, we have the responsibility to evaluate whether conditions or events raise substantial doubt about our ability to meet our future financial obligations as they become due within 12 months after the date the consolidated financial statements are issued. The ability to obtain sufficient proceeds from additional equity offerings, collaborations or other financing with terms favorable or acceptable to us cannot be considered probable, as these events are outside of our control. Based on our currently available cash resources, the $12.5 million Milestone Termination Payment received from SPN in July 2026, and the expected receipt of the additional $12.5 million Milestone Termination Payment due from SPN in October 2026, and considering our future operating plans, we anticipate that we will require additional funding following the first quarter of 2027. Accordingly, management has concluded that these circumstances raise substantial doubt about our ability to continue as a going concern. Substantial doubt about our ability to continue as a going concern may materially and adversely affect the price per share of our common stock, and it may be more difficult for us to obtain financing. If potential collaborators decline to do business with us or potential investors decline to participate in any future financings due to such concerns, our ability to increase our cash position may be limited. We will need to generate significant revenues to achieve profitability, and we may never do so. Because of the numerous risks and uncertainties associated with the development of our current and any future product candidates, the development of our platform and technology and because the extent to which we may enter into collaborations with third parties for development of any of our product candidates is unknown, we are unable to estimate the amounts of increased capital outlays and operating expenses required for completing the research and development of our product candidates.

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Six Months Ended June 30,

Nine Months Ended September 30,

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Net (decrease) increase in cash, cash equivalents and restricted cash (36,597 (33,770

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During the six nine months ended June September 30, 2026, cash used in operating activities was $38.8 $49.2 million due to a net loss of $15.3 $31.6 million and changes in our operating assets and liabilities of $38.5 $16.9 million partially offset by and non-cash charges of $15.1 $0.7 million. Non-cash charges consisted of the impairment loss of $5.8 $11.5 million, gain related to the APA Amendment of $25.0 million, stock-based compensation expense of $3.5 $5.4 million, $4.1 $5.1 million related to the amortization of right-of-use assets, $1.5 $1.9 million of depreciation and amortization, and $0.1 $0.4 million loss on disposal of fixed assets. Changes in our operating assets and liabilities during the six nine months ended June September 30, 2026 consisted of an increase in accounts receivable due from SPN of $24.6 million related to the APA Amendment, a decrease in operating lease liabilities of $14.8 million, primarily related to $10.8 $16.3 million, of termination payments related to the Lease Amendment, a decrease in accounts payable of $0.5 $0.2 million, a decrease in accrued expenses and other current and long-term liabilities of $0.1 million, and an increase in accounts receivable relating to the CARB-X program of $0.1 $2.0 million, partially offset by a decrease in prepaid expenses and other current and other non-current assets of $1.6 $1.2 million and a decrease in accounts receivable due from SPN of $0.4 million.

During the six nine months ended June September 30, 2025, cash provided by operating activities was $13.6 $15.8 million due to net income of $12.8 $21.0 million and non-cash charges of $11.4 $17.1 million, partially offset by changes in our operating assets and liabilities of $10.6 $22.3 million. Non-cash charges consisted of stock-based compensation expense of $5.2 $7.6 million, $4.1 $6.2 million related to the amortization of right-of-use assets, and $2.1 $3.2 million of depreciation and amortization. Changes in our operating assets and liabilities during the six nine months ended June September 30, 2025 consisted of a decrease in accrued liabilities due to SPN of $4.3 $13.3 million, a decrease in operating lease liabilities of $4.2 $6.4 million, a decrease in accrued expenses and other current and long-term liabilities of $4.8 $4.2 million, and a decrease in accounts payable of $1.9 $2.4 million, and an increase in accounts receivable relating to the CARB-X program of $0.4 million, partially offset by a decrease in prepaid expenses and other current and other non-current assets of $3.4 $2.7 million, and a decrease in accounts receivable due from SPN of $1.2 $1.6 million.

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During the six months ended June 30, 2026, net cash provided by investing activities was less than $0.1 million, consisting entirely of proceeds received from sales of property and equipment to unrelated third parties.

During the nine months ended September 30, 2026, net cash provided by investing activities was $12.6 million, primarily consisting of $12.5 million received from SPN related to the APA Amendment.

During the six nine months ended June September 30, 2025, net cash used in investing activities was $0.2 million, consisting entirely of purchases of property and equipment.

1 unchanged paragraph

During the six nine months ended June September 30, 2026, net cash provided by financing activities was $2.1 $2.9 million, consisting of $2.1 $2.9 million from the issuance of common stock under our at the market equity program, net of issuance costs and less than $0.1 million from the issuance of common stock under our 2015 Employee Stock Purchase Plan, or ESPP.

During the six nine months ended June September 30, 2025, net cash provided by financing activities was $1.2 million, consisting of $1.0 million from the issuance of common stock under our at the market equity program, net of issuance costs and $0.2 million from the issuance of common stock under our 2015 Employee Stock Purchase Plan, or ESPP.

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• conduct clinical trials for SER-155 in allo-HCT, irEC, and other medically vulnerable populations, including potentially SER-428 in ICU patients;

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• the cost of conducting clinical trials for SER-155 in allo-HCT, irEC, and other targeted indications, and other product candidates in our pipeline;

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As discussed in Note 1 of the condensed consolidated financial statements included elsewhere in this Quarterly Report on Form 10-Q, we have the responsibility to evaluate whether conditions or events raise substantial doubt about our ability to meet our future financial obligations as they become due within 12 months after the date the condensed consolidated financial statements are issued. The ability to obtain sufficient proceeds from additional equity offerings, collaborations or other financing with terms favorable or acceptable to us cannot be considered probable, as these events are outside of our control. Accordingly, management has concluded that these circumstances raise substantial doubt about our ability to continue as a going concern. Based on our currently available cash resources the $12.5 million Milestone Termination Payment received from SPN in July 2026, and the expected receipt of the additional $12.5 million Milestone Termination Payment due received from SPN in October 2026, and considering our future operating plans, we anticipate that we will require additional funding following the first quarter of 2027.

1 unchanged paragraph

The disclosure of our contractual obligations and commitments was included in our Annual Report. Except for the Lease Amendment and the Termination Agreement described in Note 6, Leases, to our condensed consolidated financial statements included elsewhere in this Quarterly Report on Form 10-Q there have been no material changes from the contractual commitments and obligations previously disclosed in our Annual Report.

Market risk 1 change

We are exposed to market risk from changes in interest rates and inflation. These market risks arise in the normal course of business. As of June September 30, 2026, there have been no material changes to the information included under Part II, Item 7A, Quantitative and Qualitative Disclosures About Market Risk, in our Annual Report on Form 10-K for the year ended December 31, 2025.

Controls and procedures 2 changes

3 unchanged paragraphs

Our management, with the participation of our principal executive officer and our principal financial officer, has evaluated the effectiveness of our disclosure controls and procedures (as defined in Rules 13a-15(e) and 15d-15(e) under the Securities Exchange Act of 1934, as amended), as of the end of the period covered by this Quarterly Report. Based on such evaluation, our principal executive officer and principal financial officer concluded that as of June September 30, 2026, our disclosure controls and procedures were effective at the reasonable assurance level.

1 unchanged paragraph

No change in our internal control over financial reporting (as defined in Rules 13a-15(f) and 15d-15(f) under the Exchange Act) occurred during the three months ended June September 30, 2026 that has materially affected, or is reasonably likely to materially affect, our internal control over financial reporting.

Risk factors 13 changes

3 unchanged paragraphs

Based on our currently available cash resources the $12.5 million Milestone Termination Payment received from SPN in July 2026, and the expected receipt of the additional $12.5 million Milestone Termination Payment due received from SPN in October 2026, and considering our future operating plans, we anticipate that we will require additional funding following the first quarter of 2027. The ability to obtain additional equity or other financing, including through partnerships, with terms favorable or acceptable to us cannot be considered probable according to the applicable accounting standards because they are outside our control. Therefore, there is substantial doubt about our ability to continue as a going concern for at least 12 months from the date that our condensed consolidated financial statements for the six nine months ended June September 30, 2026 were issued. Substantial doubt about our ability to continue as a going concern may materially and adversely affect the price per share of our common stock, and it may be more difficult for us to obtain financing or enter into a partnership. If potential collaborators or partners decline to do business with us or potential investors decline to participate in any future financings due to such concerns, our ability to increase our cash position may be limited. The perception that we may not be able to continue as a going concern may cause others to choose not to deal with us due to concerns about our ability to meet our contractual obligations. We have prepared our consolidated financial statements on a going concern basis, which contemplates the realization of assets and the satisfaction of liabilities and commitments in the normal course of business. Our unaudited consolidated financial statements included in this Quarterly Report on Form 10-Q do not include any adjustments to reflect the possible inability of the Company to continue as a going concern within 12 months after the issuance of such financial statements.

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• the cost of conducting clinical trials for SER-155 in allo-HCT, irEC, and other targeted indications, and other product candidates in our pipeline;

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Since inception, we have incurred significant operating losses. As of June September 30, 2026, we had cash and cash equivalents totaling $15.6 $17.5 million and an accumulated deficit of $987.7 $1,004.0 million. For the six nine months ended June September 30, 2026, we incurred a net loss from operations of $42.2 $58.3 million . As noted elsewhere in this Quarterly Report on Form 10-Q, we have identified conditions and events that raise substantial doubt about our ability to continue as a going concern. To date, we have financed our operations through the public offerings of our common stock, private placements of our common stock and preferred stock, payments under our prior collaboration agreements and loan facility, and payments from government entities for research grants. We have devoted substantially all of our financial resources and efforts to developing our live biotherapeutics platform, identifying potential product candidates and conducting preclinical studies and clinical trials. We have only developed one FDA-approved product, VOWST, which was sold to SPN in September 2024. We have not completed development of any of our other product candidates, which we call live biotherapeutic candidates, or other drugs or biologics. We expect to continue to incur significant expenses and operating losses for the foreseeable future. Our expenses may increase substantially in connection with our ongoing and future activities, particularly if and as we:

1 unchanged paragraph

• conduct clinical trials for SER-155 in allo-HCT, irEC, and for other medically vulnerable populations;

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We will need to secure additional funding to maintain operations beyond our current cash runway. Based on our currently available cash resources, the $12.5 million Milestone Termination Payment received from SPN in July 2026, and the expected receipt of the additional $12.5 million Milestone Termination Payment due from SPN in October 2026, and considering our future operating plans, we anticipate that we will require additional funding following the first quarter of 2027. However, due to our smaller business size and the early stage of development of our remaining assets, there can be no assurance that we will be able to raise the required capital on favorable terms, or at all. This potential inability to obtain necessary funding could have a material adverse effect on our growth prospects, financial condition, and results of operations.

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The UK regulatory framework in relation to clinical trials is derived from the now-repealed EU Clinical Trials Directive (as implemented into UK law, through the Medicines for Human Use (Clinical Trials) Regulations 2004, as amended). In April 2025, the UK government adopted the Medicines for Human Use (Clinical Trials) Amendment Regulations. The amendment, which will take took full effect in April 2026, aims to provide a more flexible regime to make it easier to conduct clinical trials in the UK, increase the transparency of clinical trials conducted in the UK and make clinical trials more patient centered.

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The ability of the FDA and other regulatory authorities to review and approve new products can be affected by a variety of factors, including government budget and funding levels, statutory, regulatory, and policy changes, the FDA's and other regulatory authorities' ability to hire and retain key personnel and accept the payment of user fees, and other events that may otherwise affect the FDA's and other regulatory authorities' ability to perform routine functions. Average review times at the FDA have fluctuated in recent years as a result. In addition, government funding of other government agencies that fund research and development activities is subject to the political process, which is inherently fluid and unpredictable. Disruptions at the FDA and other regulatory authorities, such as the EMA, may also slow the time necessary for new drugs and biologics to be reviewed and/or approved by necessary regulatory authorities, which would adversely affect our business. For example, in recent years, the U.S. government has shut down several times and certain regulatory authorities, such as the FDA, have had to furlough critical FDA employees and stop critical activities. In addition, the current U.S. Presidential administration has issued certain policies and Executive Orders directed towards reducing the employee headcount and costs associated with U.S. administrative agencies, including the FDA, which have led to substantial changes in senior personnel, and it remains unclear the degree to which these efforts and the resulting personnel changes may limit or otherwise adversely affect the FDA's ability to conduct routine activities.

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We believe that any of our product candidates approved as a biological product under a BLA should also qualify for the 12-year period of reference product exclusivity. However, there is a risk that this exclusivity could be shortened due to congressional action or otherwise, or that the FDA will not consider our product candidates to be reference products for competing products, potentially creating the opportunity for generic competition sooner than anticipated. Moreover, the extent to which a biosimilar, once approved, could be substituted for any one of our reference products in a way that is similar to traditional generic substitution for non-biological products will depend on a number of specific marketplace and regulatory factors that are still developing. For example, in recent years, the FDA has sought to reduce the data required for biosimilar product sponsors to demonstrate biosimilarity to, or interchangeability with, a biologic reference product, which could increase the potential for biosimilar competition.

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Our patent portfolio currently includes 20 21 active patent application families (which includes an exclusive license to certain IP from Memorial Sloan Kettering Cancer Center). Of these, 16 applications have been nationalized, three are at the PCT stage, and one is at the provisional stage. To date, we have obtained issuance of 33 U.S. patents (which includes three as licensee). Of the issued U.S. patents, 13 U.S. patents (including one as licensee) have been assigned to Nestlé Health Science as part of its purchase of VOWST. We cannot provide any assurances that any of our pending patent applications will mature into issued patents and, if they do, that such patents or our current patents will include claims with a scope sufficient to protect our product candidates or otherwise provide any competitive advantage. For example, we are pursuing claims to therapeutic, binary compositions of certain bacterial populations. Any claims that may issue may provide coverage for such binary compositions and/or their use. However, there can be no assurance that an alternative composition that may fall outside the scope of such claims will not be equally effective. Further, while our product candidates are made up of specific cultivated bacteria, third-party compositions may have greater complexity and variability (e.g., lot to lot variations), and it is possible that a patent claim may provide coverage for some but not all third-party compositions. These and other factors may provide opportunities for our competitors to design around our patents, should they issue.

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On February 27, 2026, the board of directors appointed Richard N. Kender, a current member of the board of directors, to serve as Executive Chair of the Board and as Interim Chief Executive Officer of the Company, Matthew Henn, Ph.D., the Company's Chief Scientific Officer, to the additional role of President, and Kelly Brady, M.S., the Company's Senior Vice President, Clinical Development, to the role of Executive Vice President, Chief Operating Officer, each effective March 2, 2026. (the "Effective Date"). As a result of the appointment of Mr. Kender as Interim Chief Executive Officer, Thomas J. DesRosier and Marella Thorell ceased serving as Co-Presidents and Co-Chief Executive Officers of the Company as of the Effective Date. March 2, 2026. Mr. DesRosier continues to serve as the Company's Executive Vice President and Chief Legal Officer. and On October 22, 2026, Ms. Thorell continues to serve resigned as the Company's Executive Vice President and Chief Financial Officer, effective November 15, 2026, and Mr. Kender will serve as our principal financial officer and principal accounting officer on an interim basis.

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Our executive officers, directors and stockholders who owned more than 5% of our outstanding common stock and their respective affiliates, in the aggregate, hold shares representing approximately 29% of our outstanding voting stock as of December 31, 2025. As a result, if these stockholders were to choose to act together, they would be able to significantly influence all matters submitted to our stockholders for approval, as well as our management and affairs. For example, these persons, if they choose to act together, would significantly influence the election of directors and approval of any merger, consolidation or sale of all or substantially all of our assets. This concentration of ownership control may:

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We are a "smaller reporting company" as defined under the rules promulgated under the Exchange Act. We will remain a smaller reporting company until the fiscal year following the determination that both (i) the value of our voting and non-voting common shares held by non-affiliates is more than $250.0 million measured on the last business day of our second fiscal quarter and (ii) our annual revenues are more than $100.0 million during the most recently completed fiscal year and the value of our voting and non voting nonvoting common shares held by non-affiliates is $700.0 million or more as measured on the last business day of our second fiscal quarter. Smaller reporting companies are able to provide simplified executive compensation disclosure and have certain other reduced disclosure obligations, including, among other things, being required to provide only two years of audited financial statements and not being required to provide selected financial data, or supplemental financial information.

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Our restated certificate of incorporation specifies that, unless we consent in writing to the selection of an alternative forum, the Court of Chancery of the State of Delaware will be the sole and exclusive forum for most legal actions involving actions brought against us by stockholders. In addition, our bylaws provide that the federal district courts of the United States are the exclusive forum for any complaint raising a cause of action arising under the Securities Act. Any person or entity purchasing or otherwise acquiring any interest in shares of our capital stock shall be deemed to have notice of and to have consented to the provisions of our restated certificate of incorporation and bylaws described above.

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Other information 10 changes

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Item 5.02. Departure of Directors or Certain Officers; Election of Directors; Appointment of Certain Officers; Compensatory Arrangements of Certain Officers.

As previously reported, on October 22, 2026, Marella Thorell resigned from her positions as Executive Vice President and Chief Financial Officer of the Company, effective November 15, 2026 (the "Effective Date").

On November 1, 2026, the Board appointed Richard Kender, the Company's Executive Chair and Interim Chief Executive Officer and principal executive officer, to serve as the Company's principal financial officer and principal accounting officer and succeed Ms. Thorell in these capacities, effective as of the Effective Date. Mr. Kender will continue to serve as the Company's Executive Chair and Interim Chief Executive Officer and principal executive officer, and he will not receive any additional compensation for his role as principal financial officer and principal accounting officer during this period.

A description of Mr. Kender's business experience can be found under the section entitled "Executive Officers" in the Company's definitive proxy statement on Schedule 14A, filed with the Securities and Exchange Commission on April 27, 2026, and which biography is incorporated by reference herein.

The Company and Mr. Kender have previously entered into the Company's standard indemnification agreement for executive officers. There are no family relationships between Mr. Kender and any director or executive officer of the Company that would be required to be disclosed pursuant to Item 401(d) of Regulation S-K, and Mr. Kender does not have a material interest in any transaction that is required to be disclosed under Item 404(a) of Regulation S-K.

Advisory Agreement with Marella Thorell

In connection with her resignation, on November 2, 2026, the Company and Ms. Thorell entered into an advisory agreement (the "Advisory Agreement") pursuant to which Ms. Thorell will provide advisory services to the Company from the Effective Date through March 31, 2027, unless terminated earlier by either party on 15 business days' prior written notice, in exchange for an hourly fee of $500.00. Ms. Thorell is expected to provide the Company with up to 20 hours of advisory services per month. Ms. Thorell's performance of advisory services will constitute continued service to the Company for purposes of the vesting and exercisability of her outstanding Company stock options.

Resignation of Robert Rosiello

On October 30, 2026, Robert Rosiello resigned from his position as a member of the board of directors of the Company and on the committees on which he serves, effective October 30, 2026. Mr. Rosiello's departure is not due to any disagreement with the Company on any matter relating to the Company's operations, policies or practices.

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During the three months ended June September 30, 2026, no director or "officer" (as defined in Rule 16a-1(f) under the Exchange Act) of the Company adopted or terminated a "Rule 10b5-1 trading arrangement" or "non-Rule 10b5-1 trading arrangement," as each term is defined in Item 408(a) of Regulation S-K.

Exhibits 8 changes

6 unchanged paragraphs

Non-Employee Director Compensation Program

Seres Therapeutics, Inc. 2025 Incentive Plan, as amended and restated

Amendment No. 1 to Asset Purchase Agreement, dated June 2, 2026, by and between Seres Therapeutics, Inc. and Société des Produits Nestlé S.A.

First Amendment to Lease, dated June 4, 2026, by and between Seres Therapeutics, Inc. and 101 CPD, LLC

2 unchanged paragraphs

Advisory Agreement, dated November 2, 2026, by and between Marella Thorell and Seres Therapeutics, Inc.

8 unchanged paragraphs

^ Schedules and similar attachments have been omitted pursuant to Item 601(a)(5) of Regulation S-K. The Company undertakes to furnish supplemental copies of any of the omitted schedules upon request by the U.S. Securities and Exchange Commission.

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Date: August 5, 2026

Date: November 4, 2026

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