MCRB 10-K & 10-Q changes, risk factors and insider trading

We will need additional funding in order to advance development of our product candidates (including to conduct the planned Phase 2 study of SER-155 in allo-HSCT and to advance our earlier stage programs) and commercialize our product candidates, if approved. If we are unable to raise capital or secure a partnership whenor needed,other business development transaction, we could be forcedrequired to delay,implement further cost-reduction measures, reduce or eliminatedelay our product development programs or any potential future commercialization efforts.efforts, or pursue other alternatives which could include seeking relief under the U.S. Bankruptcy Code or winding down our operations.

Our expenses may increase in connection with our ongoing activities, particularly if and as we further SER-155 clinical studies, and research, develop and initiate clinical trials of our product candidates. In addition, if we obtain marketing approval for any of our product candidates, we expect to incur costs related to product manufacturing and commercialization, including marketing, sales and distribution, and may not generate meaningful product revenues or collaboration profit in the near future. Furthermore, we have incurred and expect to continue to incur additional costs associated with operating as a public company. Accordingly, we will need to obtain substantial additional funding in connection with our continuing operations. If we are unable to raise capital or secure a partnership when needed or on attractive terms, we could be forced to delay, reduce or eliminatedelay our research andproduct development programs or any current or potential future commercialization efforts.efforts, or pursue other alternatives, which could include seeking relief under the U.S. Bankruptcy Code or winding down our operations.

the cost of developing our pipeline product candidates, including SER-603 in IBD and SER-155 in irEC the cost of conducting clinical trials for our lead candidate, SER-155 in allo-HSCT and other targeted indications, and other product candidates in our pipeline;

the total amount of the Second Installment Payment and Milestone Payments we may receive from the Transaction, and the amounts payable or due under the Profit Sharing PaymentsTransaction;

Any additional fundraising efforts may divert our management from their day-to-day activities, which may adversely affect our ability to develop and commercialize our product candidates. In addition, we cannot guarantee that future financing will be available in sufficient amounts or on terms acceptable to us, if at all. Additionally, market volatility resulting from current macroeconomic conditions, such as the conflicts involving Ukraine and Russia and Israel and its surrounding regions, trade wars or related uncertainty between the U.S. and other nations, including China, or other governmental action related to tariffs or international trade agreements or policies, and related impacts, or other factors could also adversely impact our ability to access capital as and when needed. Moreover, the terms of any financing may adversely affect the holdings or the rights of our stockholders and the issuance of additional securities, whether equity or debt, by us, or the possibility of such issuance, may cause the market price of our shares to decline. The sale of additional equity or convertible securities would dilute all of our stockholders and may decrease our stock price. The incurrence of indebtedness could result in increased fixed payment obligations and we may be required to agree to certain restrictive covenants, such as limitations on our ability to incur additional debt, limitations on our ability to acquire, sell, or license intellectual property rights and other operating restrictions that could adversely impact our ability to conduct our business. We could also be required to seek funds through arrangements with collaborators or others at an earlier stage than otherwise would be desirable and we may be required to relinquish rights to some of our technologies or product candidates or otherwise agree to terms unfavorable to us, any of which may have a material adverse effect on our business, operating results and prospects.

If we are unable to obtain funding on a timely basis, we may be required to significantly curtail, delay, or discontinue one or more of our research or development programs or any product candidates, or be unable to expand our operations or otherwise capitalize on our business opportunities, as desired, which could materially adversely affect our business, financial condition and results of operations. We may pursue strategic partnerships or collaborations or other alternatives with one or more parties. We can provide no assurance that we will be successful in securing any strategic partnership, collaboration, or alternative, or that any such partnership, collaboration or alternative that we secure will achieve its intended benefits or have a positive impact on our financial condition or business.

Since inception, we have incurred significant operating losses. Our net loss from continuing operations was $125.8$94.0 million, $190.1$121.3 million, and $183.6$195.1 million for the years ended December 31, 2025, 2024, 2023, and 2022,2023, respectively. As of December 31, 2024,2025, we had cash and cash equivalents totaling $45.8 million and an accumulated deficit of $978.1$972.4 million. As noted elsewhere in this Annual Report on Form 10-K, 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.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. While we plan to focus our investment on continuing the development of SER-155 and advancing our other wholly-owned cultivated live biotherapeutic candidates, ourOur expenses may increase substantially in connection with our ongoing and future activities, particularly if and as we:

continue theconduct clinical developmenttrials offor SER-155 in patients receiving allo-HSCT and for other medically vulnerable populations;

The total amount of the Second Installment Payment and Milestone Payments we may receive from the Transaction,Transaction and the amounts payable or due under the Profit Sharing Payments, areis subject to various risks and uncertainties.

In connection with the Closing, SPN assumed certain liabilities with respect to the VOWST Business and agreed to pay to usus, among other payments:

cash installment payments of $50 million, which was received on January 15, 2025, and $25 million, which was received July 1, 2025, (offset by $1.4 million duepaid by us to Nestlé on July 1, 2025, or the Second Installment Payment (to be reduced by approximately $1.5 million2025 related to certain employment obligations assumed by SPN, as described below), conditioned on our material compliance with obligations under the TSA entered into at Closing between us and NESA;

As they are earned, the Milestone Payments will be satisfied as follows: (1) first, by set-off against all accrued interest on the Prepaid Milestone, (2) second, by set-off against the outstanding balance of the Prepaid Milestone until the Prepaid Milestone has been repaid in full and (3) 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, 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 Secondinstallment Installmentpayment Payment duereceived on July 1, 2025 willwas be reducedoffset by approximately $1.5$1.4 million we paid to Nestlé on the same date related to certain employment obligations assumed by SPN through the period prior to the Closing Date. Further, during the Profit Sharing Period, we and SPN shared 50/50 in the net profit or net loss achieved during the period.

The Second Installment Payment and the Milestone Payments are subject to various risks and uncertainties. We must be in material compliance with our obligations under the TSA in order to receive the Second Installment Payment and, if we are not or if there is a dispute as to compliance, such payment could be withheld or delayed, pending resolution. The Milestone Payments will beare based on the achievement of specified worldwide net sales targets for the Product. Interest on the Prepaid Milestone will accrue and will reduce any corresponding Milestone Payments based on the length of time it takes to achieve the milestones. It is not possible to determine with precision as of the date of this Annual Report on Form 10-K the amount or timing of worldwide net sales the Product will generate in the future and, therefore, it is possible that certain of the Milestone Payments will not be earned or will be limited by lower Product net sales than anticipated. The specified worldwide net sales targets for the Product were based on certain assumptions about the future financial performance of the Product, and there can be no assurance that such projections will be achieved or that certain of the Milestone Payments will become payable.

We will need to secure additional funding to maintain operations beyond our current cash runway. Based on our currently available cash resources, including the capital obtained from the Transaction, and the expected receipt of the Second Installment Payment, which is subject to material compliance with the TSA, and considering our future operating plans and our ongoing obligations related to the Transaction,plans, we anticipate that we will require additional funding infollowing the firstthird quarter of 2026. 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.

We were obligated to use the proceeds from the completion of the Transaction to fully repay our indebtedness under theour prior credit facility with Oaktree CreditCapital Agreement.Management ("Oaktree"). We have broad discretion with respect to the use of the remaining proceeds of the Transaction, including to support the further advancement of SER-155 and our other cultivated live biotherapeutic product candidates. The results and effectiveness of the use of proceeds are uncertain, and we could spend the proceeds in ways that do not improve our remaining business, financial condition or results of operations. Our failure to apply these funds effectively could have an adverse effect on its business, financial condition and results of operations.

We are early in our development efforts of certain of our product candidates and may not be successful in our efforts to use our reverse translational platform to build a pipeline of product candidates and develop additional marketable drugs.

It is currently unclear to what extent the UK will seek to align its regulations with the EU. 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). TheIn extentApril to which the regulation of clinical trials in the UK will mirror the (EU) CTR in the long term is not yet certain, however, on December 12, 2024,2025, the UK government introduced a legislative proposal -adopted the Medicines for Human Use (Clinical Trials) Amendment RegulationsRegulations. 2024The -amendment, that, if implemented,which will replacetake thefull current regulatory framework for clinical trialseffect in theApril UK. The legislative proposal2026, 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. The UK government has provided the legislative proposal to the UK Parliament for its review and approval. Once the legislative proposal is approved (with or without amendment), it will be adopted into UK law which is expected in early 2026. A decision by the UK government not to closely align any new legislation with the new approach that has been adopted in the EU may have an effect on the cost of conducting clinical trials in the UK as opposed to other countries.

The process of obtaining marketing approvals, both in the United States and abroad, is expensive, risky and may take many years. The scope and amount of clinical data required to obtain marketing approvals can vary substantially from jurisdiction to jurisdiction, and it may be difficult to predict whether a particular regulatory body will require additional or different studies than those conducted by a sponsor, especially for novel product candidates such as our live biotherapeutic candidates. The FDA or foreign regulatory authorities may delay, limit, or deny approval to market our product candidates for many reasons, including: our inability to demonstrate that the clinical benefits of our product candidates outweigh any safety or other perceived risks; the regulatory authority’s disagreement with the interpretation of data from nonclinical or clinical studies; the regulatory authority’s requirement that we conduct additional preclinical studies and clinical trials; changes in marketing approval policies during the development period; changes in or the enactment of additional statutes or regulations, or changes in regulatory review process for each submitted product application; or the regulatory authority’s failure to approve the manufacturing processes or third-party manufacturers with which we contract. For instance, the EU pharmaceutical legislation ishas currentlybeen undergoing a complete review process, in the context of the Pharmaceutical Strategy for Europe initiative, launched by the European Commission in November 2020. The European Commission'sCommission’s proposal for revision of several legislative instruments related to medicinal products was published on April 26, 2023. The proposed changes were since discussed and negotiated by the European Parliament and the Council of the EU as part of the EU ordinary legislative process. A provisional agreement has been reached by the European Parliament and Council of the EU on the proposed revisions on December 11, 2025. The proposed revisions (potentially reducingaffecting the duration of regulatory data protection and market protection, including for orphan medicinal products, revising the eligibility for expedited pathways, etc.) was published on April 26, 2023. The proposed revisions remain to be agreed andformally adopted by the Europeantwo Parliament and European Council and the proposals may therefore be substantially revised before adoption,institutions, which is not anticipated before early 2026. The revisionsproposed changes are not expected to enter into application before 2028 and may however have a significant impact on the biopharmaceutical industry in the long term.

Disruptions at the FDA and other government agencies caused by funding shortagesshortages, staffing limitations or globalpolicy health concernschanges could hinder their ability to hire, retain or deploy key leadership and other personnel, or otherwise prevent new or modified products from being developed, approved or commercialized in a timely manner or at all, which could negatively impact our business.

The ability of the FDA and other regulatory authorities to review and or 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, and it remains unclear the degree to which these efforts may limit or otherwise adversely affect the FDA's ability to conduct routine activities.

Separately, in response to the COVID-19 pandemic, the FDA postponed most inspections of domestic and foreign manufacturing facilities at various points. If a prolonged government shutdown occurs, or if renewedfunding globalshortages, healthstaffing concernslimitations, or changes in administrative policy delay or prevent the FDA or other regulatory authorities from conducting their regular inspections, reviews, or other regulatory activities, it could significantly impact the ability of the FDA or other regulatory authorities to timely review and process our regulatory submissions, which could have a material adverse effect on our business. It could also impact our ability to access the public markets and obtain necessary capital in order to fund our operations.

Recently enactedCurrent and future legislation or regulation may increase the difficulty and cost for us to obtain marketing approval of and commercialize our product candidates and may adversely affect the prices we may obtain.

In the United States and some foreign jurisdictions, there have been a number of legislative and regulatory changes and proposed changes regarding the healthcare system that could prevent or delay marketing approval of our product candidates, restrict or regulate post-approval activities and affect our ability to profitably sell any product candidates for which we obtain marketing approval. We expect that current laws, as well as other healthcare reform measures that may be adopted in the future, including executive orders, may result in more rigorous coverage criteria and in additional downward pressure on the price that we may receive for any product candidates approved for sale. New and changing laws and regulations may also create uncertainty about how such laws and regulations will be interpreted and applied. If we are found to have violated laws and regulations, it could materially adversely affect our business, results of operations and financial condition.

Among the provisions of the ACA of importance to our otherbusiness, potentialincluding without limitation, our ability to commercialize and the prices we may obtain for any product candidates that are approved for sale are the following:

extension of manufacturers’ Medicaid rebate liability to apply to Medicaid managed care utilization;

aexpansion newof requirementthe toentity annuallytypes reporteligible for participation in the 340B drug samplespricing that manufacturers and distributors provide to physiciansprogram; and a new Patient-Centered Outcomes Research Institute to oversee, identify priorities in and conduct comparative clinical effectiveness research, along with funding for such research.

Since its enactment, there have been judicial, executive and Congressional challenges to certain aspects of the ACA. On June 17, 2021, the U.S. Supreme Court dismissed the most recent judicial challenge to the ACA brought by several states without specifically ruling on the constitutionality of the ACA. Thus, the ACA will remain in effect in its current form.

In addition, other legislative changes have been proposed and adopted since the ACA was enacted. For example, the Budget Control Act of 2011, enacted in August 2011, required sequestration that included aggregate reductions of Medicare payments to providers, which went into effect on April 1, 2013 and, due to subsequent legislative amendments, will remain in effect through 2032, unless additional Congressional action is taken. Under current legislation, the actual reduction in Medicare payments will increase in future years of the sequester. OnIn January 2, 2013, the American Taxpayer Relief Act of 2012 was signed into law, which, among other things, reduced Medicare payments to several providers, including hospitals, and an increase in the statute of limitations period for the government to recover overpayments to providers from three to five years.

Further, in March 2021, the American Rescue Plan Act of 2021 was signed into law, which, among other things, eliminated the statutory cap on drug manufacturers’ Medicaid Drug Rebate Program rebate liability, effective January 1, 2024. Drug manufacturers’ Medicaid Drug Rebate Program rebate liability was previously capped at 100% of the average manufacturer price for a covered outpatient drug. We expect that other healthcare reform measures that may be adopted in the future may result in additional reductions in Medicare and other healthcare funding, more rigorous coverage criteria, new payment methodologies and in additional downward pressure on the price that we receive for any approved product. Any reduction in reimbursement from Medicare or other government programs may result in a similar reduction in payments from private payors. The implementation of cost containment measures or other healthcare reforms may prevent us from being able to price our product candidates, if and when they are approved, at what we consider to be a fair or competitive price, generate revenue, attain profitability, or commercialize our product candidates, if approved.

Moreover, there has recently been heightened governmental scrutiny over the manner in which manufacturers set prices for their marketed products. Individual states in the United States have become increasingly active in implementing regulations designed to contain pharmaceutical and biological product pricing, including price or patient reimbursement constraints, discounts, restrictions on certain product access and marketing cost disclosure and transparency measures. Most significantly, on August 16, 2022, President Biden signed theThe Inflation Reduction Act of 2022,Act, or the IRA, intowas law.enacted in 2022. This statute marks the most significant action by Congress with respect to the pharmaceutical industry since adoption of the ACA in 2010. Among other things, the IRA requires manufacturers of certain drugs to engage in price negotiations with Medicare (beginning in 2026),Medicare, with prices that can be negotiated subject to a cap; imposes rebates under Medicare Part B and Medicare Part D to penalize price increases that outpace inflation (first due in 2023); redesigns the Medicare Part D benefit (which began in 2024); and replaces the Part D coverage gap discount program with a new manufacturer discounting program (which began in 2025). CMS has published the negotiated prices for the initial ten drugs, which went into effect in January 2026, and the subsequent 15 drugs, which will first be effective in 2027. CMS has also published the next set of 15 drugs that will be subject to negotiation. The IRA permits the Secretary of the Department of Health and Human Services, or HHS, to implement many of these provisions through guidance, as opposed to regulation, for the initial years. HHS has and will continue to issue and update guidance as these programs are implemented. The Centers for Medicare & Medicaid Services, or CMS, has published the negotiated prices for the initial ten drugs, which will first be effective in 2026, and has published the list of the subsequent 15 drugs that will be subject to negotiation,implemented, although the Medicare drug price negotiation program is currently subject to legal challenges. LegallyThe mandatedimpact priceof controlsthe IRA on paymentus amountsand bythe third-partypharmaceutical payorsindustry orcannot otheryet restrictionsbe couldfully harmdetermined, ourbut abilityis likely to price our product candidates, if and when they are approved, appropriately, which could negatively impact our business, results of operations, financial condition and prospects. In addition, regional healthcare authorities and individual hospitals are increasingly using bidding procedures to determine what pharmaceutical products and which suppliers will be included in their prescription drug and other healthcare programs. This could reduce the ultimate demand for our product candidates, if approved, or put pressure on our product pricing, which could negatively affect our business, results of operations, financial condition and prospects.significant.

Our patent portfolio currently includes 2122 active patent application families (which includes an exclusive licenses to certain IP from Memorial Sloan Kettering Cancer Center). Of these, 1918 applications have been nationalized, one is at the PCT stage, and onethree isare at the provisional stage. To date, we have obtained issuance of 3133 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.

We are highly dependent on Eric Shaff, our PresidentInterim and Chief Executive Officer,CEO, as well as the other principal members of our management, scientific and clinical team. Although we have entered into employment agreements with our executive officers, each of them may terminate their employment with us at any time. We do not maintain “key person” insurance for any of our executives or other employees.

Recruiting and retaining qualified scientific, clinical, manufacturing and sales and marketing personnel will also be critical to our success. The loss of the services of our executive officers or other key employees could impede the achievement of our research, development and potential future commercialization objectives and seriously harm our ability to successfully implement our business strategy. Furthermore, replacing executive officers and key employees may be difficult and may take an extended period of time because of the limited number of individuals in our industry with the breadth of skills and experience required to successfully develop, gain regulatory approval of and commercialize products. Competition to hire from this limited pool is intense, and we may be unable to hire, train, retain or motivate these key personnel on acceptable terms given the competition among numerous pharmaceutical and biotechnology companies for similar personnel. We also experience competition for the hiring of scientific and clinical personnel from universities and research institutions. In addition, we rely on consultants and advisors, including scientific and clinical advisors, to assist us in formulating our research and development and commercialization strategy and execution. Our consultants and advisors may be employed by employers other than us and may have commitments under consulting or advisory contracts with other entities that may limit their availability to us. If we are unable to continue to attract and retain high quality personnel, our ability to pursue our growth strategy will be limited.

We currently have limited international operations, but our business strategy incorporates potentially expanding internationally if any of our product candidates receive regulatory approval. We have conducted clinical studies in Australia and New Zealandinternationally in the past, and will likely in the future conduct clinical studies in other countries as well. Doing business internationally involves a number of risks, including but not limited to:

A discussion regarding our financial condition and results of operations for the years ended December 31, 20242025 and 2023,2024, including a year-to-year comparison between 20242025 and 2023,2024, is presented below. For a full discussion of changes from the year ended December 31, 2024 to the year ended December 31, 2023, refer to Management’s Discussion and Analysis of Financial Condition and Results of Operation in Part II, Item 7. of our Annual Report on Form 10-K for the year ended December 31, 2024 (filed with the SEC on March 13, 2025).

Our live biotherapeuticLBP candidates are consortia of bacteria designed to optimize specific, targeted pharmacological properties, and are formulated for oral delivery. We maintain a differentiated live biotherapeutics drug discovery and development platform that includes good manufacturing practices, or GMP, manufacturing capabilities for this novel drug modality. Our pipeline assets, including SER-155 and SER-603, are designed to target multiple disease-relevant pathways and are manufactured from standard clonal cell banks via cultivation. Our knowledge base and platforms enable selection of bacteria for precision consortia design to drive specific clinical outcomes and further provide unique insights on microbe-associated disease targets. We are designing live biotherapeuticLBP candidates optimized to prevent the colonization and overgrowth of pathogens in the gastrointestinal tract and modulate host function to increase epithelium integrity and to induce immune tolerance. We believe clinicalhomeostasis and nonclinicaltolerance, dataas acrosswell ouras programsto supportprevent the developmentcolonization and overgrowth of livepathogens biotherapeutics to targetin the preventiongastrointestinal, andor treatmentGI, of a broad swath of infections, and in inflammatory and immune diseases.tract. We believe that the scientific and clinical data from the development of VOWST (our then product candidate SER-109 program) and the data from the SER-155 Phase 1b study (described below) validate thisour novel therapeutic approach in both infectious disease and I&I diseases. In the context of infection.infection, Wewe believe thisthat approachour technology may be replicable across different bacterial pathogens with the potential to develop live biotherapeutics with the potential to protect a range of medically compromised patients fromat risk of antimicrobial resistance, or AMR, infections and bloodstream infections, includingor pathogensBSIs, that harborcan AMR.result from a compromised epithelial barrier and that can be a major cause of mortality.

InWe additionbelieve to allo-HSCT, we intend to evaluatethat SER-155 and other cultivated live biotherapeutic candidates could be developed in other medically vulnerableadditional patient populations to address barrier compromise and bloodstream and AMR infections beyond allo-HSCT, including autologous-HSCT patients, cancer patients with neutropenia, chimeric antigen receptors therapytherapy, or CAR-T, recipients, individuals with chronic liver disease, or CLD, solid organ transplant recipients, as well as patients in the intensive care unitunit, or ICU, and long-term acute care facilities. AdditionalWe effortscontinue to develop another proprietary live biotherapeutic composition, SER-147, designed to prevent bacterial bloodstream and spontaneous bacterial peritonitis, or SBP, infections in patients with metabolic disease, including CLD. Additionally, we are developing an oral liquid formulation based on SER-155 strains, for dosing in patients who cannot take oral capsules, such as intubated patients in the early-stageICU, portfolioand areother focusedmedically onvulnerable thepatients SER-301at programhigh inrisk inflammatoryof bowelAMR disease, or IBD, with programmatic objectivesinfections, supported throughby a partnershipgrant withfrom the Crohn’s and Colitis Foundation, or CCF. These efforts aim toCARB-X (iCombating Antibiotic-Resistant Bacteria Biopharmaceutical Accelerator). confirm the functional phenotype and inflammatory state of patient subpopulations observed in our prior ulcerative colitis, or UC, clinical trials, and (ii) prioritize inflammatory targets and evaluate the potential to utilize biomarker-based patient selection and stratification for future studies. In addition, weWe continue to leverage microbiome pharmacokinetic and pharmacodynamic data from across our clinical and preclinical portfolios, using our reverse translational development platform to prioritize future drug targets and to identify opportunities for monotherapy treatment and in combination with existing therapies across various indications, including inflammatory and immune diseases, cancer, and metabolic diseases.

We have built and deploy a reverse translational platform and knowledge base, which we call our MbTx Platform, 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 microbiological capabilities and a strain library and associated microbiological capabilities that spans broad biological and functional breadth. This platform and knowledge base enablesare 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.

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 continuing operations was $125.8$94.0 million, $190.1$121.3 million, and $183.6$195.1 million for the years ended December 31, 2025, 2024, 2023, and 2022,2023, respectively. As of December 31, 2024,2025, we had an accumulated deficit of $978.1$972.4 million.

While we plan to focus our investment on advancing our early-stage live biotherapeutic candidates, starting with SER-603, in the near-term and, subject to receiving additional funding, progressing the development of SER-155 andinto advancinga ourPhase other2 wholly-owned live biotherapeutic candidates in the near-term,study, our expenses may increase in connection with these future activities. See “Risk Factors—Risks Related to Our Financial Position and Need for Additional Capital—We are a clinical-stage company and have incurred significant losses since our inception. We expect to incur losses for the foreseeable future and may never achieve or maintain profitability.”

As a result, we will need additional financing to support our continuing operations. Until such time as we can generate significant revenue from product sales, if ever, we expect to finance our operations through a combination of collaborations with third parties, public or private equity or debt financings or other sources. Adequate additional financing may not be available to us on acceptable terms, or at all. For example, the trading prices for our and other biopharmaceutical companies’ stock have been highly volatile as a result of factors such as the impacts of pandemics, such as COVID-19, and increases in inflation rates orrates, interest rates.rates and tariffs. As a result, we may face difficulties raising capital through sales of our common stock and any such sales may be on unfavorable terms. Our inability to raise capital or secure a partnership as and when needed would have a negative impact on our financial condition and our ability to pursue our business strategy. We will need to generate significant revenue to achieve profitability, and we may never do so.

As of December 31, 2024,2025, we had cash and cash equivalents totaling $30.8$45.8 million. Based on our currently available cash resources, including the capital obtained from the Transaction, and the expected receipt of the Second Installment Payment, which is subject to material compliance with the TSA, and considering our future operating plans and our ongoing obligations related to the Transaction,plans, we anticipate that we will require additional funding infollowing the firstthird quarter of 2026. 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 consolidated financial statements included elsewhere in this Annual Report on Form 10-K. In performing this analysis, we excluded certain elements of our operating plan that cannot be considered probable of occurring. Under the applicable accounting standards, the receipt of contingent payments from the Transaction and 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 consolidated financial statements included elsewhere in this Annual Report on Form 10-K, 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.”

We continue to be invested in the infectious disease space, with a renewed focus on leveraging our existing clinical data, translational insights, and manufacturing capabilities to support targeted development efforts across a defined set of related indications. We believe that the scientific and clinical data from our VOWST program (our then product candidate SER-109 program) validate our novel approach of using live biotherapeutics to decolonize pathogens and improve epithelial barrier integrity, resulting in reduced rate of infections in medically compromised patients. Data from the SER-109 ECOSPOR III and ECOSPOR IV Phase 3 trial published in the New England Journal of Medicine (Feuerstadt et al., 2022) and Journal of the American Medical Association (Sims et al., 2023) suggest that live biotherapeutics have the potential to restructure the gut microbiome and shift the gut metabolic landscape. Additional data show that SER-109VOWST rapidly reduced the abundance of bacteria associated with common antibiotic resistance genes, or ARGs, and reduced ARG abundance in the gut (Straub et al., 2023). Collectively, we believe these data suggest the potential for live biotherapeutics to prevent the colonization and overgrowth of pathogens that can establish in the gut and ultimately to reduce infections. We believe that reducing pathogen colonization in the GI and improving GI epithelial barrier integrity to reduce the risk of infection may be replicable in a range of medically compromised patients, protecting them from infections and resulting downstream clinical sequelae. We believe this approach may also enable us to reduce antimicrobial resistant infections, or AMR, which the World Health Organization declared as a top ten global public health threat facing humanity, and with estimates that yearly deaths may reach 10 million by 2050, putting mortality due to AMR on par with deaths due to cancer.

SER-155 Phasein 1b Studyallo-HSCT

SER-155 Phase 1b Study (including placebo-controlled Cohort) SER-155 has been evaluated in a Phase 1b study in patients undergoing allo-HSCT. The SER-155 Phase 1b study included two cohorts. Cohort 1 was designed to assess safety and drug pharmacology, specifically the drug strain engraftment in the gastrointestinalGI tract. Cohort 1 included 13 subjects who received any dosing of the SER-155 regimen, with 11 subjects subsequently receiving an allo-HSCT. Results from this cohort, announced in May 2023, showed SER-155 was generally well tolerated and resulted in successful drug strain engraftment and a reduction in pathogen domination in the GI microbiome relative to a historical control cohort.

The median age in Cohort 2 was 63, and most subjects had acute myeloid leukemia, acute lymphocytic leukemia, myelodysplastic syndrome or myeloproliferative neoplasia as their primary disease and received reduced-intensity conditioning pre-transplant. Most patients received peripheral blood stem cells from a matched unrelated donor. A majority received post-transplant cyclophosphamide as part of their graft-versus-host disease (GvHD) prophylaxis.

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 thirdfirst quarter of 2026.2027. 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 threesix months ended MarchJune 31,30, 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.

We will need additional funding in order to advance development of our product candidates (including to conduct the Phase 2 study of SER-155 in allo-HSCTallo-HCT and to advance our earlier stage programs) and commercialize our product candidates, if approved. If we are unable to raise capital or secure a partnership or other business development transaction, we could be required to implement further cost-reduction measures, reduce or delay our product development programs or any potential future commercialization efforts, or pursue other alternatives which could include seeking relief under the U.S. Bankruptcy Code or winding down our operations.

We are actively seeking a business development transaction, which may include a partnership, to provide financial support and share in our vision to realize the clinical and commercial value of SER-155. Our strategic discussions are focused on supporting SER-155 clinical advancement to reduce the risk of bloodstream infections, including life-threatening and AMR infections, in medically vulnerable patient populations. We believe that SER-155 and other cultivated live biotherapeutic candidates could be developed in additional patient populations beyond allo-HSCT,allo-HCT, including autologous-HSCTautologous-HCT patients, cancer patients with neutropenia, CAR-T recipients, individuals with chronic liver disease, solid organ transplant recipients, as well as patients in the intensive care unit and long-term acute care facilities. We are also exploring collaborations to advance the development of our investigational biotherapeutics in inflammatory and immune diseases, including ulcerative colitis and Crohn’s disease.

the cost of developing our pipeline product candidates, including SER-603 in IBD and SER-155 in irEC the cost of conducting clinical trials for SER-155 in allo-HSCT and other targeted indications, and other product candidates in our pipeline;

Since inception, we have incurred significant operating losses. As of MarchJune 31,30, 2026, we had cash and cash equivalents totaling $29.8$15.6 million and an accumulated deficit of $992.3$987.7 million. For the threesix months ended MarchJune 31,30, 2026, we incurred a net loss from operations of $19.9$42.2 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:

invest in our early-stage pipeline product candidates, including SER-603 in IBD and SER-155 in irEC conduct clinical trials for SER-155 in allo-HSCT and for other medically vulnerable populations;

Since our inception in October 2010, we have devoted substantially all of our resources to developing our clinical and preclinical program, building our intellectual property portfolio, developing our supply chain, planning our business, raising capital and providing general and administrative support for these operations. Other than with respect to VOWST, which was sold to SPN in September 2024, we have not yet demonstrated our ability to obtain regulatory approvals, and we have limited experience in demonstrating our ability to manufacture a commercial-scale product, or arrange for a third party to do so on our behalf, or conduct sales and marketing activities necessary for successful product commercialization. Additionally, we expect our financial condition and operating results to continue to fluctuate significantly from quarter to quarter and year to year due to a variety of factors, including for example, the impact of the sale of our VOWST Business to SPN, many of which are beyond our control. Consequently, any predictions made about our future success or viability may not be as accurate as they could be if we had a longer operating history.

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 thirdfirst quarter of 2026.2027. 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.

In addition, many of the factors that cause, or lead to, the termination or suspension of, or a delay in the commencement or completion of, clinical trials may also ultimately lead to the denial of regulatory approval of a product candidate. We do not know whether any of our preclinical studies or clinical trials will begin as planned, will need to be restructured or will be completed on schedule, or at all. Significant preclinical or clinical trial delays also could shorten any periods during which we may have the exclusive right to commercialize our product candidates or allow our competitors to bring products to market before we do, potentially impairing our ability to successfully commercialize our product candidates and harming our business and results of operations.

In addition, the FDA’s and other regulatory authorities’ policies may change and additional government regulations may be enacted with respect to clinical trials. For instance, the regulatory landscape related to clinical trials in the European Union, or EU, recently evolved. The EU Clinical Trials Regulation, or CTR, which was adopted in April 2014 and repeals the EU Clinical Trials Directive, became applicable on January 31, 2022. While the EU Clinical Trials Directive required a separate clinical trial application, or CTA, to be submitted in each member state in which the clinical trial takes place, to both the competent national health authority and an independent ethics committee, the CTR introduces a centralized process and only requires the submission of a single application for multi-center trials. The CTR allows sponsors to make a single submission to both the competent authority and an ethics committee in each member state, leading to a single decision per member state. The assessment procedure of the CTA has been harmonized as well, including a joint assessment by all member states concerned, and a separate assessment by each member state with respect to specific requirements related to its own territory, including ethics rules. Each member state’s decision is communicated to the sponsor via the centralized EU portal. Once the CTA is approved, clinical study development may proceed. The CTR transition period ended on January 31, 2025, and all clinical trials (and related applications) are now fully subject to the provisions of the CTR. Compliance with the CTR requirements by us and our third-party service providers, such as contract research organizations, or CROs, may impact our developmentsdevelopment plans.

From time to time, we may publicly disclose interim,Interim, top-line or preliminary data from our preclinical studies and clinical trials, which is based on a preliminary analysis of then-available data, and the results and related findings and conclusions are subject to change following a more comprehensive review of the data related to the particular study or trial. We also make assumptions, estimations, calculations and conclusions as part of our analyses of data, and we may not have received or had the opportunity to fully and carefully evaluate all data. As a result, the top-line or preliminary results that we report may differ from future results of the same studies, or different conclusions or considerations may qualify such results, once additional data have been received and fully evaluated. Top-line or preliminary data also remain subject to audit and verification procedures that may result in the final data being materially different from the top-line or preliminary data we previously published. As a result, top-line and preliminary data should be viewed with caution until the final data are available. Adverse differences between interim data and final data could significantly harm our business prospects. Further, disclosure of interim data by us or by our competitors could result in volatility in the price of our common stock.

We have and may in the future seek Fast Track designation for some of our product candidates. If a drug or biologic is intended for the treatment of a serious or life-threatening condition and nonclinical or clinical data demonstrate the potential to address unmet medical needs for this condition, the drug or biologic sponsor may apply for Fast Track designation. We received Fast Track designation for SER-155 to reduce the risk of infection and GvHD in patients undergoing allo-HSCT.allo-HCT. Fast Track designation applies to the combination of the product candidate and the specific indication for which it is being studied. Once granted, Fast Track designation provides increased opportunities for sponsor meetings with the FDA during preclinical and clinical development, and a BLA submitted for a Fast Track product candidate may also be eligible for rolling review, where the FDA may consider for review sections of the BLA on a rolling basis before the complete application is submitted, if the sponsor provides a schedule for the submission of the sections of the BLA, the FDA agrees to accept sections of the BLA and determines that the schedule is acceptable, and the sponsor pays any required user fees upon submission of the first section of the application.

In December 2024, we received Breakthrough Therapy designation for SER-155 for the reduction of BSIs in patients 18 years and older undergoing allo-HSCT.allo-HCT. We may seek these or other designations for future product candidates. A Breakthrough Therapy is defined as a drug or biologic that is intended to treat a serious or life-threatening disease or condition, and preliminary clinical evidence indicates that the drug or biologic may demonstrate substantial improvement over existing therapies on one or more clinically significant endpoints, such as substantial treatment effects observed in early clinical development. For drugs or biologics that have been designated as breakthrough therapies, interaction and communication between the FDA and the sponsor can help to identify the most efficient path for clinical development. Drugs designated as breakthrough therapies by the FDA also receive all of the Fast Track program features, including eligibility for rolling review of the associated marketing application.

The ability of the FDA and other regulatory authorities to review and or 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, and it remains unclear the degree to which these efforts may limit or otherwise adversely affect the FDA's ability to conduct routine activities.

If we obtain approval orof any of our product candidates, we may face competition from biosimilars. In the United States, the Biologics Price Competition and Innovation Act, or BPCIA, enacted in 2010 as part of the Patient Protection and Affordable Care Act, created an abbreviated approval pathway for biological products that are demonstrated to be “highly similar,” or biosimilar, to or “interchangeable” with an FDA-approved biological product. Under the BPCIA, an application for a biosimilar product may not be submitted to the FDA until four years following the date that the reference product was first licensed by the FDA. In addition, the approval of a biosimilar product may not be made effective by the FDA until four years from the date on which the reference product was first licensed. During this 12-year period of exclusivity, another company may still market a competing version of the reference product if the FDA approves a full BLA for the competing product containing the sponsor’s own preclinical data and data from adequate and well-controlled clinical trials to demonstrate the safety, purity and potency of their product. This pathway could allow competitors to reference data from innovative biological products 12 years after the time of approval of the innovative biological product, though the FDA may not approve an application relying on such data for a further eight years. This data exclusivity does not prevent another company from developing a product that is highly similar to the innovative product, generating its own data and seeking approval. Data exclusivity only assures that another company cannot rely upon the data within the innovator’s application to support the biosimilar product’s approval.

the False Claims Act, imposes, among other things, impose criminal and civil penalties, including through civil whistleblower or qui tam actions, against individuals or entities for knowingly presenting, or causing to be presented, to the federal government, claims for payment that are false or fraudulent, knowingly making, using or causing to be made or used, a false record or statement material to a false or fraudulent claim or from knowingly making a false statement to avoid, decrease or conceal an obligation to pay money to the federal government. In addition, the government may assert that a claim including items or services resulting from a violation of the federal Anti-Kickback Statute constitutes a false or fraudulent claim for purposes of the federal False Claims Act;

Our patent portfolio currently includes 2120 active patent application families (which includes an exclusive license to certain IP from Memorial Sloan Kettering Cancer Center). Of these, 1816 applications have been nationalized, twothree are at the PCT stage, and twoone areis 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.

We have significant excess leased space that may adversely affect our financial condition and results of operations.

Following the sale of our VOWST Business, the completion of the TSA, and our cost reduction actions, we have significantly reduced our workforce and manufacturing footprint. AsIn aJune result,2026, we entered into the First Amendment to Lease with respect to our space at 101 Cambridge Park Drive, Cambridge, Massachusetts, which reduced our leased space from approximately 82,714 to 36,882 rentable square feet. In July 2026, we entered into the Third Amendment to Lease and Termination Agreement with respect to our space at 200 Sidney Street, Cambridge, Massachusetts, which provided for the surrender of approximately 21,295 rentable square feet and the revision of the lease expiration date for the remaining approximately 47,341 rentable square feet from January 2031 to December 31, 2026. We continue to have excess capacity under our existingremaining lease obligations that exceeds our current and anticipated operational needs. We remain obligated to make rental and other payments under these leases regardless of whether we utilize the leased space. Our near-term lease obligationsobligations, including under these lease amendments require us to make substantial cash payments that reduce the cash available to fund our research and development programs, clinical trials, and other operational needs.

We may seek to sublease the excess space to reduce our ongoing costs; however, there can be no assurance that we will be able to identify suitable subtenants or negotiate sublease arrangements on favorable terms, or at all. Current commercial real estate market conditions, including elevated vacancy rates and declining demand for certain types of office and laboratory space, may makefurther itlimit difficultany sublease opportunities that remain available to sublease our excess space or may require us to accept sublease terms that do not fully offset our lease obligations.us. Even if we are able to sublease theany excess space, we would remain primarily liable to the landlord under the underlying leases, and any default by a subtenant could result in additional costs and liabilities to us.

In addition, we have been required to, and may in the future be required to recognize impairment charges with respect to right-of-use assets associated with leased properties that are no longer being fully utilized. Any such impairment charges could have a material adverse effect on our results of operations and financial condition.

The costs associated with our excessremaining leasedlease space,obligations, termination payments, and impairment charges, combined with any inability to sublease such space or potential impairment charges, could materially and adversely affect our liquidity, cash flows, financial condition, and results of operations, and could limit our ability to invest in the advancement of SER-155 and our other pipeline programs.

We are subject to complex and changing laws, regulations, and executive orders, and compliance with these laws and regulations and executive orders is onerous and expensive. New and changing laws, regulations, and executive orders can adversely affect our business by increasing our costs, limiting the Company’s ability to pursue or offer a product candidate or product, and requiring changes to our business. New and changing laws, regulations, and executive orders can also create uncertainty about how such laws and regulations will be interpreted and applied. Regulatory changes and other actions that materially adversely affect our business may be announced with little or no advance notice and we may not be able to effectively mitigate all adverse impacts from such measures. Differing interpretations of such legal obligations can expose us to significant fines, government investigations, litigation and reputational harm. If we are found to have violated laws, regulations, or executive orders, it could materially adversely affect our business, reputation, results of operations and financial condition.

our requirement for additional capital to fund our operations following the thirdfirst quarter of 20262027;

Additionally, we and/or other parties in our value chain are subject to, or are expected to be subject to additional climate and other ESG-related obligations arising from legislation and regulation in the United States, the European Union and other jurisdictions, including new reporting requirements, even as the availability and quality of the information that may be required to comply with such laws and regulations remains limited. We expect for our compliance costs with these laws, regulations, and reporting requirements to increase in the future, and any failure, or perceived failure, by us to adhere to such laws, regulations, and reporting requirements, or meet evolving and varied stakeholder expectations and standards, could harm our business, reputation, financial condition, and operating results.

We are a clinical-stage company focused on improving patient outcomes in medically vulnerablecompromised populationspopulations, including oncology patients and those suffering from inflammatory and immune, or I&I, diseases through discovery and development of novel live biotherapeutic product, or LBP, candidates. We led the successful development and approval of VOWST, the first FDA-approved orally administered microbiome biotherapeutic and a Breakthrough Therapy designated drug, which was sold to Société des Produits Nestlé S.A., or SPN, and with certain of its affiliates, collectively, Nestlé Health Science, in September 2024. We have established field-leading capabilities and platforms that are powered by best-in-field human data sets to advance a portfolio of products that can uniquely address diseases by targeting host pathways that are modulated by microbes in the human body, and, in particular, diseases associated with mucosal epithelial barrier-immune interface targets. We believe clinical and nonclinical data across our programs support the development of LBPs to targetaddress unmet medical needs in oncology that can lead to interruption to patients' cancer treatment and/or mortality (by targeting the prevention and treatment of a broad swath of infections,infection risks faced by cancer patients and by treating irEC), and to treat inflammatory and immune, or I&I,I diseases. Our pipeline consists of SER-155, SER-603, SER-428, and SER-147, as well as other potential candidates in earlier development.

We are designing LBP candidates to modulate host function to increase epithelium integrityprotect and toimprove mucosal epithelial barrier integrity, induce immune homeostasis and tolerance, as well as toand prevent the colonization and overgrowth of pathogens in the gastrointestinal, or GI, tract. We believe that the scientific and clinical data from the development of VOWST (our then product candidate SER-109 program) and the data from the SER-155 Phase 1b study in allo-HSCTallo-HCT and the SER-155 IST in irEC (both further described below) validate our novel therapeutic approach to address unmet medical needs in bothoncology infectiousthat diseasecan lead to interruption to patients' cancer treatment and/or mortality, and to treat I&I diseases. In the context of infection, 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 epithelial barrier and that can be a major cause of mortality.

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-HSCT.allo-HCT. In our placebo-controlled Phase 1b study of SER-155 in allo-HSCT,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 HSCT.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. Following advancement of key startup activities for the SER-155 Phase 2 study in allo-HSCT,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.

Our current strategy prioritizes advancing our programs that target I&I indications. We have meaningfully advanced over the past decade our scientific understanding of how microbes in the GI functionally modulate pathways at the mucosal epithelial barrier-immune interface that are associated with inflammatory and immune-related disease. The clinical data from our SER-155 Phase 1b study in allo-HSCT,allo-HCT and the IST in irEC, in particular the drug pharmacology translational biomarker data, along with our extensive preclinical and translational clinical data compiled over the past decade support and inform the advancement of our earlier stage programs targeting I&I diseases.diseases and our current strategy continues to prioritize these indications. We are evaluating SER-155 in immune checkpoint-related enterocolitis, or 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. 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. 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. We expect to report initial clinical results, including preliminary safety, efficacy, pharmacology, and exploratory biomarker data in the coming weeks. We believe data from this IST could further support the potential for live biotherapeutics to address a significant unmet need among the large population of cancer patients receiving ICIs and may further support evaluation of our biotherapeutic approach in this setting. We are also developing SER-603, broadly in inflammatory bowel disease, or IBD, including ulcerative colitis, or UC, and Crohn's disease. SER-603 is a novel, LBP candidate optimized to address disruptions in the GI microbiome and to improve GI mucosal epithelial barrier integrity through the inhibition of inflammatory bacteria and associated metabolites, the promotion of epithelial barrier integrity to reduce the translocation of inflammatory molecules and barrier inflammation, and to induce immune homeostasis through non-immunosuppressive regulatory T cell, or T-reg, induction via T cell signaling. We believe that our LBPs could represent a non-immunosuppressive treatment option for I&I diseases that are linked to colitis and could broadly address immune therapy toxicities, both of which represent significant unmet medical needs and potential commercial opportunities. We are currently exploring potential collaborations related to those I&I disease programs.

We believe that SER-155 and other cultivated live biotherapeutic candidates could be developed in additional patient populations toin addresswhich mucosal epithelial barrier compromise andis implicated. For example, in bloodstream and AMR infectionsinfections, there are numerous medically compromised patients beyond allo-HSCT,allo-HCT, including autologous-HSCTautologous-HCT patients, cancer patients with neutropenia, chimeric antigen receptors therapy, or CAR-T, recipients, individuals with chronic liver disease, or CLD, solid organ transplant recipients, as well as patients in the intensive care unit, or ICU, and long-term acute care facilities. In 2025, we received a non-dilutive award from CARB-X (Combating Antibiotic-Resistant Bacteria Biopharmaceutical Accelerator) for up to $3.6 million, representing our second CARB-X award, to support the development of an oral liquid formulation based on SER-155 strains, now referred to as SER-428, for dosing in patients who cannot take oral capsules such as intubated patients in the medical ICU, and other medically vulnerable patients at high risk of AMR infections. We have advanced development and manufacturing of SER-428 and are designing a Phase 1b open label trial in collaboration with Columbia University to evaluate this LBP candidate in medical ICU patients at high risk of infection. Additionally, we continue to develop another proprietary live biotherapeutic composition, SER-147, designed to prevent bacterial bloodstream and spontaneous bacterial peritonitis, or SBP, infections in patients with metabolic disease, including CLD. We continue to leverage microbiome pharmacokinetic and pharmacodynamic data from across our clinical and preclinical portfolios, using our reverse translational development platform to prioritize future drug targets and to identify opportunities for monotherapy treatment and in combination with existing therapies across various indications, including inflammatory and immune diseases, cancer, and metabolic diseases.

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 $19.9$42.2 million for the threesix months ended MarchJune 31,30, 2026. As of MarchJune 31,30, 2026, we had an accumulated deficit of $992.3$987.7 million.

While weWe plan to focus our investment in the near-term on advancing our early-stage live biotherapeutic candidates, startingincluding withadvancing IND-enabling studies for SER-603, inevaluating the near-term,next stage of development for SER-155 in irEC, and, subject to receiving additional funding, progressing the development of SER-155 into a Phase 2 study in allo-HSCT,allo-HCT. ourOur expenses may increase in connection with these future activities. See “Risk Factors—Risks Related to Our Financial Position and Need for Additional Capital—We are a clinical-stage company and have incurred significant losses since our inception. We expect to incur losses for the foreseeable future and may never achieve or maintain profitability.”

As of MarchJune 31,30, 2026, we had cash and cash equivalents totaling $29.8$15.6 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 thirdfirst quarter of 2026.2027. 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.”

On February 12, 2026, we announced that we implemented cost reduction actions, including reducing our workforce by approximately 30%. During the three months ended March 31, 2026, we incurred approximately $0.9 million in restructuring costs, primarily related to severance costs, of which $0.7 million was paid asduring the first quarter of March2026, 31,and 2026. Thethe remainder of these costs are expected to bewas paid out byin the second quarter of 2026.

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 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 2120 active patent families, which includes 1816 nationalized applicationsapplications, three at the PCT stage, and twoone 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.

Research and development activities are central to our business model. Product candidates in later stages of clinical development generally have higher development costs than those in earlier stages of clinical development, primarily due to the increased size and duration of later-stage clinical trials. We anticipate an overall decrease in research and development expenses in 2026 as compared to 2025, following the conclusion of the TSA, completion of the Phase 1b study of SER-155 in allo-HSCT,allo-HCT, and the cost reduction actions that were implemented in September 2025 and February 2026, including pausing investment in the SER-155 Phase 2 study and headcount reductions. Research and development expenses may increase in the future if and as we resume development of any clinical or preclinical programs. In 2025, research and development expenses included labor and passthrough costs, reimbursable by Nestlé, incurred in performing obligations under the TSA. Given the conclusion of the TSA as of December 31, 2025, these costs and related reimbursements are not expected to reoccur in 2026.

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 months ended MarchJune 31,30, 2026 or 2025.

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 months ended MarchJune 31,30, 2026.

Comparison of Three Months Ended MarchJune 31,30, 2026 and 2025

The following table summarizes our results of operations for the three months ended MarchJune 31,30, 2026 and 2025:

Total revenue was $0.4$0.7 million for the three months ended MarchJune 31,30, 2026 consisting of costs reimbursable under the CARB-X grant that we were awarded in the third quarter of 2025.

Research and development expenses were $13.2$9.1 million for the three months ended MarchJune 31,30, 2026 and $11.8$12.9 million for the three months ended MarchJune 31,30, 2025. The increasedecrease of $1.4$3.8 million was primarily due to the following: