Engineered beneficial bacteria reduce bloodstream infections after stem cell transplants. Researchers at City of Hope(R), one of the largest and most advanced cancer research and treatment organizations in the country, and collaborators have pioneered a new approach to preventing dangerous bloodstream infections in patients receiving donor stem cell transplants for blood cancers. The first-of-its-kind therapy delivers 16 carefully selected beneficial bacteria designed to restore the microbiome and fight harmful pathogens. In the multicenter phase 1 clinical trial, scientists from City of Hope, Memorial Sloan Kettering Cancer Center and Seres Therapeutics found that the new therapy, called SER-155, was well tolerated with no treatment-related side effects. Patients who received the therapy also had fewer bloodstream infections and lower levels of harmful bacteria linked to infection. The Seres Therapeutics sponsored study, published today in Nature Medicine, provides early evidence that live biotherapeutics - medicines made from beneficial bacteria - can be safely delivered to highly vulnerable patients and may help prevent serious infections, said senior author Marcel van den Brink, M.D., Ph.D., City of Hope chief physician executive and the Morgan and Helen Chu Presidential Chair of City of Hope National Medical Center. "This is a landmark study for the field," Dr. van den Brink said. "The immediate result is fewer bloodstream infections, but the larger significance is what it tells us about the future of medicine. For the first time, we've shown that it's possible to design a consortium of beneficial bacteria with a specific clinical purpose and to safely deliver it to highly vulnerable patients. That opens the door to an entirely new class of therapies." Treating the microbiome. The microbiome - the trillions of bacteria and other microbes that live in and on the body - is increasingly recognized as a key factor in cancer care. Researchers are finding that it can influence how patients respond to treatment, recover from therapy or develop side effects. City of Hope is helping to turn those discoveries into new ways to prevent complications, improve treatment outcomes and support recovery. For this nationwide clinical trial, researchers focused on patients undergoing allogeneic stem cell transplantation, an effective but high-risk treatment for leukemia and other blood cancers. While the treatment can be lifesaving, it leaves patients highly vulnerable to infection as their immune systems recover. Bloodstream infections are among the most serious complications and can lead to sepsis, a life-threatening condition. Currently, patients undergoing transplants are given antibiotics to prevent infection. However, there are no therapies available that treat the underlying disruption of the microbiome that makes patients vulnerable to infection in the first place. Event guide: 10th CAR-TCR Summit eBook Navigate the latest developments in cell therapy at the 10th CAR-TCR Summit Europe. Download the latest edition Fecal transplants have been used to restore healthy gut bacteria in other conditions that disrupt the microbiome, like c. difficile infection. But these treatments are like a grab-bag of bacteria - it can be hard to know what strains they actually contain. That makes them potentially dangerous for immunocompromised patients. A targeted therapy using bacteria. For the new live biotherapeutic, researchers carefully identified and selected strains of bacteria that would offer targeted, infection-preventing benefits. These included bacteria that are known to eat up the nutrients used by bad bacteria, release pathogen-killing compounds or regulate the body's immune response. "Instead of giving patients a broad mix of bacteria, we selected specific strains for specific jobs and put them into a single pill," Dr. van den Brink said. "That level of precision is what makes this approach fundamentally different from simply trying to replenish the microbiome. We're moving from observing the microbiome to engineering it for specific therapeutic outcomes." In the phase 1 clinical trial, researchers first treated patients with an antibiotic to clear their existing microbiomes and make it easier for the introduced bacteria to establish new ecosystems. They then gave patients pills containing the selected bacteria. Treatment was given before transplant and repeated at least once after transplant, since many patients are given antibiotics to prevent infection. Because the treatment involves giving live bacteria to severely immunocompromised patients, a key question was whether the approach was safe. Researchers showed that it was. "The safety profile was excellent - we didn't see any complications with infections," Dr. van den Brink said. The study also showed that the therapy worked as intended. Researchers found that the good bacteria successfully established themselves in the microbiome, especially after the second course of treatment. Although the study wasn't focused on clinical outcomes, early results were promising. Patients who received the therapy had lower levels of the most harmful pathogens and significantly lower risk of bloodstream infections. Beyond the encouraging results, Dr. van den Brink noted that the study is important because it demonstrates the potential for scientifically designed live biotherapeutic drugs. "We are only beginning to understand the therapeutic potential of the microbiome," he said. How cancer patients can support their microbiome. Larger clinical trials are needed before the new therapy can be approved for use and made available to patients. However, research shows there are other strategies that can help support the microbiome during cancer treatment, Dr. van den Brink said. These include: * Practice antibiotic stewardship: Patients should discuss the responsible use of antibiotics with their doctors, including avoiding the use of broad-spectrum antibiotics, which can disrupt the microbiome and promote antibiotic resistance. * Eat a high-fiber diet: "Prebiotics," like whole grains, fruits, vegetables and other high-fiber foods, provide nutrients for good bacteria, supporting a healthy microbiome. "Diet is a drug," he said. "Fiber and fiber-rich diets are shown to be a benefit in every setting where it has been studied." * Avoid probiotics: Over-the-counter probiotic capsules contain a mix of bacteria that may be dangerous for immuno-compromised patients. In some cases, these capsules may contain strains of bacteria that studies have linked to worse outcomes in cancer, Dr. van den Brink noted. City of Hope's microbiome program is working on follow-up research, including studies and clinical trials to develop more therapies targeting the microbiome as a strategy for improving immunotherapy. Journal reference: Be the first to rate this article

Seres Therapeutics announces publication of SER-155 Phase 1b results in Nature Medicine showing reduced bloodstream infections in allogeneic hematopoietic cell transplantation recipients. Peer-reviewed publication reports a 77% relative risk reduction in bloodstream infections and significantly lower systemic antibiotic exposure with SER-155 compared with placebo Translational data from the study support the SER-155 mechanisms of strengthening the mucosal epithelial barrier and reducing systemic inflammation CAMBRIDGE, Mass., Nov. 03, 2026 (GLOBE NEWSWIRE) - Seres Therapeutics, Inc. (Nasdaq: MCRB), a leading live biotherapeutics company, today announced that results from its Phase 1b clinical study of SER-155 in adults undergoing allogeneic hematopoietic cell transplantation (allo-HCT) have been published in the November 3, 2026 issue of Nature Medicine. SER-155 is an investigational, oral, cultivated multi-strain live biotherapeutic product (LBP) designed to reduce the risk of bloodstream infections (BSIs) in medically vulnerable patients. The publication reports that SER-155 was generally well tolerated and was associated with a 77% relative risk reduction in BSIs compared with placebo. SER-155 has received Breakthrough Therapy designation from the U.S. Food and Drug Administration (FDA) for the reduction of BSIs in adults undergoing allo-HCT, as well as Fast Track designation, and the Company has aligned with the FDA on its proposed Phase 2 study protocol. SER-155 is Phase 2-ready in allo-HCT, pending receipt of funding. "Publication in Nature Medicine is an important milestone for the SER-155 program and recognition of the innovative science being advanced by Seres," said Matthew Henn, Ph.D., President and Chief Scientific Officer of Seres Therapeutics. "Many bloodstream infections in transplant patients originate in the gut, when pathogens expand and cross a damaged intestinal barrier. SER-155 was designed to address both sides of that problem by preventing the colonization and expansion of pathogens, many that can harbor antibiotic resistance, in the gastrointestinal tract that results from extensive antibiotic use in these patients and also protecting and improving mucosal epithelial barrier integrity that is disrupted by conditioning cancer therapy to limit bacterial translocation from the GI into the bloodstream and potential mortality due to infection. In this study, SER-155 reduced bloodstream infections while improving biomarkers of barrier integrity and systemic inflammation, providing clinical validation of these mechanisms. We believe our live biotherapeutic approach could be highly differentiated in multiple diseases where loss of gut mucosal epithelial barrier integrity is thought to play a central role. In addition, SER-155 has the potential to address the continued emergence of antimicrobial resistant infections, particularly those in immunocompromised patients, and we thank CARB-X for their prior support of this program, our collaborators and the patients who participated in this clinical study." The Phase 1b study (NCT04995653) evaluated SER-155 in adults undergoing allo-HCT for hematologic malignancies. Cohort 1 was an open-label cohort that assessed safety and drug pharmacology, including engraftment of SER-155 strains in the gastrointestinal (GI) tract. Cohort 2 was a randomized, double-blind, placebo-controlled cohort that evaluated safety, drug pharmacology and clinical outcomes, including BSIs, systemic antibiotic exposure and febrile neutropenia, through day 100 after transplant. In Cohort 2, BSIs occurred in 10% of patients receiving SER-155 compared with 42.9% of patients receiving placebo, a 77% relative risk reduction (p=0.0423). Patients receiving SER-155 also had significantly lower systemic antibiotic exposure, with a mean of 9.2 days compared with 21.1 days for placebo (p=0.0494), and a numerically lower incidence of febrile neutropenia (65% compared with 78.6%). SER-155 strains engrafted in the GI tract, and SER-155 was generally well tolerated, with no treatment-related serious adverse events observed. Translational biomarker analyses from the study further support SER-155 mechanisms of action. Compared with placebo, SER-155 led to a statistically significant decrease in fecal albumin, an established biomarker of epithelial barrier integrity, and lower plasma levels of biomarkers of systemic inflammation. SER-155 also improved biomarkers of immune homeostasis. Improved mucosal epithelial barrier integrity may reduce bacterial translocation from the GI tract into the bloodstream, consistent with the reduction in BSIs observed in the study. These barrier-protective and anti-inflammatory effects also support the potential of Seres' LBPs in inflammatory and immune diseases, including immune checkpoint inhibitor related enterocolitis (irEC) and inflammatory bowel disease (IBD). "These results, now peer-reviewed and published in Nature Medicine, reinforce our belief in the potential of SER-155 to prevent bloodstream infections in immunocompromised patients like allo-HCT patients," said Richard Kender, Executive Chairman and Interim Chief Executive Officer of Seres Therapeutics. "We believe the ability of SER-155 to strengthen the gut barrier and reduce inflammation gives it potential well beyond transplant, including in immune checkpoint inhibitor-related enterocolitis (irEC), a serious complication of cancer immunotherapy where an immunosuppressive-free treatment could allow patients to stay on their cancer therapy." Seres is also developing SER-155 for irEC, a frequent and often severe adverse reaction that can force patients to interrupt or discontinue immune checkpoint inhibitor (ICI) cancer therapy and initiate immunosuppressive therapy. In July 2026, Seres reported results from an open-label investigator-sponsored trial at Memorial Sloan Kettering Cancer Center in 15 patients with Grade 2-3 irEC who had not received immunosuppressive therapy. Twelve of 15 participants (80%) achieved an immunosuppressive-free clinical response at day 15, the study's primary endpoint, and SER-155 was generally well tolerated, with no serious adverse events assessed as related to SER-155. In the irEC study, fecal calprotectin and fecal albumin, markers of gut inflammation and mucosal barrier integrity, decreased significantly from baseline by day 43, consistent with the barrier-protective activity observed in allo-HCT. Publication Details Journal: Nature Medicine Title: A live biotherapeutic product for adult allogeneic hematopoietic cell transplantation recipients: a partially randomized, placebo-controlled phase 1b trial Publication Date & Access: November 3, 2026 Authors: Doris M. Ponce*, Zachariah DeFilipp*, Christopher B. Ford, Bina Tejura, Mary-Jane Lombardo, David I. Lichter, Meghan Chafee, Satyajit Kosuri, Nandita Khera, Hemant S. Murthy, Karamjeet S. Sandhu, David N. Fredricks, Tessa Andermann, Rajneesh Nath, Kelly Brady, Brooke R. Hasson, Emily Walsh, Nathan Hicks, Jonathan U. Peled, Lisa von Moltke, Matthew R. Henn, Marcel R.M. van den Brink**, Dennis M. Walling** and the SER-155-001 Study Team***. *These authors contributed equally. **These authors jointly supervised this work. ***A list of consortium authors and their affiliations appears at the end of the paper. About Seres Therapeutics Seres Therapeutics, Inc. (Nasdaq: MCRB) is a clinical-stage biotechnology company developing novel live biotherapeutics products in oncology that address causes of mortality and discontinuation of life-saving cancer treatment, and in inflammatory and immune (I&I) diseases, by modulating host function to protect and improve mucosal epithelial barrier integrity, induce immune homeostasis and tolerance, and prevent the colonization and overgrowth of pathogens in the gastrointestinal (GI) tract. The Company previously led the development and FDA approval of VOWST(TM), the first orally administered microbiome therapeutic, which was subsequently divested to Nestlé Health Science. SER-155, an investigational cultivated multi-strain biotherapeutic, which has received Breakthrough Therapy and Fast Track designations, is being advanced for the prevention of bloodstream infections in patients undergoing allogeneic hematopoietic stem cell transplant (allo-HCT), and is Phase 2 ready, pending receipt of funding. SER-155 is also being developed to prevent immune checkpoint inhibitor-related enterocolitis (irEC), with the aim of enabling patients to continue their ICI cancer therapy and avoid the need for immunosuppressive steroid treatment. Following promising results from an investigator-sponsored trial, Seres plans to submit the design of a study for the prevention of irEC in patients initiating ICI therapy to the FDA. The Company is advancing IND-enabling studies for SER-603, which is in development for inflammatory bowel disease. Mechanistically, Seres' biotherapeutics target the mucosal epithelial barrier-immune interface and are optimized to modulate host function to increase mucosal epithelium integrity, induce immune homeostasis, and prevent the colonization and overgrowth of harmful bacteria in the GI tract. For more information, please visit www.serestherapeutics.com. About CARB-X CARB-X (Combating Antibiotic-Resistant Bacteria Biopharmaceutical Accelerator) is the world's leading non-profit partnership accelerating the research and development of innovative products to prevent, diagnose and treat bacterial infections and address the global threat of antimicrobial resistance. CARB-X pairs non-dilutive funding with comprehensive technical and scientific support, resulting in supported projects being five times more likely to advance into clinical development than comparable projects that did not receive CARB-X support. Research reported in this press release is partially supported by CARB-X. CARB-X is funded in part with federal funds from the U.S. Department of Health and Human Services (HHS); Administration for Strategic Preparedness and Response; Biomedical Advanced Research and Development Authority (BARDA) under agreement number 75A50122C00028 and by awards from Wellcome (WT224842), the UK Department of Health and Social Care's Global Antimicrobial Resistance Innovation Fund (GAMRIF), the Gates Foundation, Germany's Federal Ministry of Research, Technology and Space (BMFTR), the Novo Nordisk Foundation, Italy's Ministry of Economy and Finance (MEF), Japan's Ministry of Health, the European Commission's DG Health Emergency Preparedness and Response Authority (DG HERA), and KfW Development Bank. The U.S. National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH) in HHS, provides support in the form of in-kind services through access to a suite of preclinical services for product development. The content of this press release is solely the responsibility of the authors and does not necessarily represent the official views of CARB-X or its funders Forward-Looking Statements This press release contains forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. All statements contained in this press release that do not relate to matters of historical fact should be considered forward-looking statements, including, without limitation, statements about: SER-155 and its intended uses and benefits in allo-HCT and irEC; our clinical development plans for SER-155; the timing and results of clinical studies and data readouts; current or future product candidates and their potential impacts and outcomes; our ability to obtain funding or enter into partnerships; the anticipated timing of any of the foregoing; and other statements that are not historical fact. These forward-looking statements are based on management's current expectations. These statements are neither promises nor guarantees, but 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, including, but not limited to, the following: (1) our need for additional funding; (2) our ability to continue as a going concern; (3) we have incurred significant losses, are not currently profitable and may never become profitable; (4) our cost reduction actions may not achieve their intended benefits, including an extended cash runway; (5) our limited operating history; (6) we may not be able to realize the anticipated benefits of the VOWST sale, and may face new challenges as a smaller, less diversified company; (7) we have in the past and may in the future receive notice of the failure to satisfy a continued listing rule from The Nasdaq Stock Market LLC; (8) our novel approach to therapeutic intervention; (9) our reliance on third parties to conduct our clinical trials and manufacture our product candidates; (10) our ability to achieve market acceptance necessary for commercial success; (11) the competition we will face; (12) our ability to protect our intellectual property; (13) impact of our recent management transitions and appointments and our ability to retain key personnel; and (14) disruptions at the FDA or other government agencies. These and other important factors discussed under the caption "Risk Factors" in our Quarterly Report on Form 10-Q for the quarterly period ended June 30, 2026, filed with the Securities and Exchange Commission (SEC), as well as our other reports filed with the SEC, could cause actual results to differ materially from those indicated by the forward-looking statements made in this press release. Any such forward-looking statements represent management's estimates as of the date of this press release. While we may elect to update such forward-looking statements at some point in the future, except as required by law, we disclaim any obligation to do so, even if subsequent events cause our views to change. These forward-looking statements should not be relied upon as representing our views as of any date subsequent to the date of this press release.