Patients and Physicians

Seres is working to develop safe, effective, oral therapies for C. difficile, ulcerative colitis and other serious diseases.

Seres’ microbiome therapeutics represent a transformative new approach to medicine

In recent years, scientific research has revealed the many essential roles played by the human microbiome — that is, the trillions of microbes that live in or on your body.

The gastrointestinal microbiome, specifically, has been found to play a central role in human health.

Conversely, a growing body of evidence also indicates that individuals who are missing certain microbiome-provided functions are at an increased risk for diseases such as C. difficile infection and ulcerative colitis, or may respond poorly to treatments for other diseases, such as cancer.

Seres’ microbiome therapeutics are designed to modulate key functions by altering the makeup of the gut microbiome. Our oral investigational therapeutic candidates are composed of live bacteria that can induce durable change in the microbiome.

A healthy microbiome performs numerous beneficial functions throughout the body.

C. difficile infection

Seres has announced positive topline results in the Phase 3 ECOSPOR III study of SER-109 in the treatment of C. difficile. Patients may still enroll in an open-label study. Learn more:  

Clostridioides difficile, formerly known as Clostridium difficile, or C. difficile, is one of the top three most urgent bacterial threats in the United States, according to the Centers for Disease Control. It is the leading cause of hospital-acquired infection in the U.S. and is responsible for the deaths of approximately 20,000 Americans each year.

The current standard of care for C. difficile infection is antibiotic treatment. However, antibiotic treatment alone is ineffective against dormant spore forms of C. difficile, which can germinate and grow after antibiotics are completed, leading to a vicious cycle of recurrence. Repeated antibiotic exposure also depletes beneficial bacteria that inhibit C. difficile, increasing the risk of recurrence.

SER-109 is an investigational oral microbiome therapeutic for the prevention of recurrent C. difficile infection. The FDA has granted SER-109 both Breakthrough Therapy and Orphan Drug designations.

Seres has announced positive topline results in the Phase 3 ECOSPOR III study of SER-109 in the treatment of recurrent C. difficile. Seres is currently enrolling subjects in the SERES-013 ECOSPOR IV open-label study for SER-109.  All individuals that qualify for the study and participate will receive SER-109, the active study drug. To learn more about the study and to find out if you or a loved one may be eligible to participate, please visit:

Please note that Seres is not currently accepting direct requests from patients for Expanded Access / Compassionate Use of SER-109 for recurrent C. difficile infection. Clinical trials offer the safest and best opportunity for people to access an investigational medicine before it is approved by a regulatory agency. Clinical trial participants receive quality care, since the health care providers managing the clinical trial closely monitor participants and all aspects of their disease. All patients who have recurrent C. difficile infection and are interested in participating in the SER-109 open-label study should contact Seres and receive a referral to one of the sites conducting our open-label study.

Please contact us via this form, and a member of the Seres team will contact you with information on the closest available participating study center. Or you can email us at

Please note that this is not a sign-up form or a waiting list for any clinical studies.

Currently SER-109 is only being administered to patients with recurrent C. difficile infection at our sites conducting the SERES-013 open-label clinical trial ( identifier: NCT03183141). If you are a physician interested in referring patients to a SERES-013 clinical site near you or are seeking information on the use of SER-109 for recurrent C. difficile infection you may request more information by contacting Seres at

Receipt of a request for information will be acknowledged within 3 business days.

A member of the Seres team will contact you with information on how you may refer your patients to a Principal Investigator at an active clinical site on the 013 open-label study.

If you are a physician interested in becoming a participating PI on the 013 trial, please contact Seres at, and a member of the team will contact you.

SER-109 is Seres’ investigational microbiome therapeutic candidate for the treatment of recurrent C. difficile infection. SER-109 is composed of bacteria that are designed to modulate the following functions in the gut microbiome to treat C. difficile infection:


Now enrolling: A Phase 1b study in Australia and New Zealand for patients with mild-to-moderate ulcerative colitis. Learn more: 

Ulcerative colitis (UC) is a serious chronic condition marked by persistent inflammation in the digestive tract which can greatly reduce an individual’s quality of life. Although the pathogenesis of UC remains unclear, changes in the gastrointestinal microbiome and associated metabolites appear to be important. The relationship between the immune system and the gut microbiome also appears to play a role in the disease.

There is a clear need for new treatments for patients with mild-to-moderate UC. Anti-inflammatory compounds like 5-ASAs are used to reduce inflammation in mild-to-moderate UC patients, but they fail to induce remission in a significant proportion of patients. Corticosteroids are used to dampen the immune system and control flare-ups, but they carry toxicity risks that limit their long-term use. Patients who do not respond to early-stage treatments are likely to advance to biologic drugs and small molecules, which bring a high risk of side effects and may require IV delivery.

Seres has completed enrollment of a clinical trial of SER-287, an oral consortium of commensal bacteria designed to have pharmacological effects on the pathways driving UC.

Seres is now enrolling patients with mild-to-moderate ulcerative colitis in a Phase 1b study of SER-301 in Australia and New Zealand. Learn more:

If you have any questions about Seres’ clinical trials, please contact us at:

Phone: 617-945-9626, extension 304

SER-287 is Seres’ groundbreaking investigational microbiome therapeutic for the treatment of UC. SER-287 is composed of bacteria designed to modulate the following functions of the gut microbiome to treat UC:

Seres’ Phase 1b study of SER-287 in individuals with mild-to-moderate UC who were failing current therapies found that SER-287 had a dose-dependent impact on disease remission. Learn more about Phase 1b results of SER-287.

SER-301 is Seres’ investigational fermented microbiome therapeutic for the treatment of mild-to-moderate UC. SER-301 is a consortium of multiple bacterial strains that is manufactured by fermenting each strain individually and then combining to form drug product. It is designed to modulate the following microbiome functions to treat UC:

Metastatic melanoma

Cancer treatments that work by turning the immune system against cancer, such as checkpoint inhibitors and other immunotherapies, only work for an estimated 20-30% of treated patients. One factor in a patient’s response appears to be the makeup of his or her gut microbiome. In collaboration with MD Anderson Cancer Center and Memorial Sloan Kettering Cancer Center, Seres has identified a microbiome signature that correlates with patients’ response to therapy across a number of different types of cancer.


Gopalakrishnan et al. Intervention strategies for microbial therapeutics in cancer immunotherapy. IOTECH. 2020 May 20; (awaiting formal publication) doi:10.1016/j.iotech.2020.05.001

Conference Talks and Posters:

Leveraging gut microbiota networks to impact tumor immunotherapy
American Association for Cancer Research conference, May 2019

Leveraging gut microbiota to impact tumor immunotherapy
American Association for Cancer Research Conference, April 2018

Frequently asked

Seres’ microbiome therapeutics are consortia of human commensal bacteria empirically selected for their combined pharmacological effects, and their safety and efficacy are being evaluated in human clinical trials registered with the U.S. Food and Drug Administration (FDA). Our manufacturing and quality teams rigorously demonstrate the viability of the bacteria in our therapeutics, and their ability to survive stomach acidity during delivery to the colon. Our clinical studies have demonstrated that the bacteria in our investigational therapeutics grow and populate in the gut and drive changes in the microbiome composition and modulate function through the generation of microbe-associated metabolites.

Probiotics, by contrast, are sourced from food, soil or stool. Most probiotics do not survive stomach acidity, and some are not typically found in the gastrointestinal tract. Probiotic lots can have wide variability in content and quality because they do not have to meet regulatory requirements for therapeutics. Some studies have shown that probiotic capsules have dead bacteria or no contents at all. Probiotics can be sold under the claim of benefit for “digestive health,” but most rigorous clinical trials of probiotics have not demonstrated therapeutic benefit.

Fecal microbiota transplantation, or FMT, involves the transfer of minimally processed donor stool into the patient’s gastrointestinal tract. Few high-quality, placebo-controlled clinical studies of FMT have been conducted, limiting the understanding of FMT efficacy and safety. Additionally, because the use of FMT for the treatment of recurrent C. difficile infection is proceeding under FDA Enforcement Discretion, FMT is not subject to the rigorous safety monitoring and oversight that are standard requirements for investigational drugs. Recent FDA safety alerts reporting hospitalizations and deaths following the administration of FMT containing pathogens such as drug-resistant E. coli have underscored the risk of transmission by FMT. Further, recent guidance from the FDA on the possible transmission of SARS-CoV-2 (the virus that leads to COVID-19) via FMT highlights additional risks of transmission of unknown or emerging pathogens by minimally processed stool. At Seres Therapeutics, our scientific platform has enhanced our understanding of the essential components needed for a therapeutic effect while mitigating risk to patients.

Seres’ microbiome therapeutics, in contrast to FMT, are being evaluated in controlled clinical studies with FDA oversight. Each study is conducted under an Investigational New Drug application (IND) with rigorous safety monitoring and reporting procedures. Our microbiome therapeutics are comprised of highly purified consortia of bacteria manufactured under Good Manufacturing Practices (GMP) conditions and quality-controlled to stringent standards.

While FMT is minimally-processed donor stool containing bacteria, fungi and viruses, Seres utilizes protective processes in manufacturing to enhance safety. Whether Seres manufactures using fermentation of pure bacterial strains which are then combined to produce the therapeutic (where only therapeutic bacterial strains are present, for example SER-301 and SER-155) or a comprehensive purification process (SER-109, for example), the risk of potential pathogens, including pathogenic non-spore bacteria, parasites and viruses, is eliminated or significantly mitigated, even where diagnostic assays may not yet be available, such as for emerging infectious diseases.

Our therapeutic candidates are comprised of highly purified consortia of spore-based commensal bacteria, which are manufactured under Good Manufacturing Practices conditions and quality controlled using stringent standards to ensure product quality and consistency. In addition to rigorous donor screening for SER-109 and SER-287, Seres utilizes a unique manufacturing process that has been demonstrated to inactivate numerous potential pathogens. Seres believes that donor screening and product testing are necessary but insufficient to minimize risks of donor-derived microbiome and FMT products. Seres’ additional manufacturing steps to inactivate potential pathogens represent a critical intervention that increases quality assurance and minimizes the risk to patients.



Our therapeutic candidates are comprised of highly purified consortia of spore-based commensal bacteria, which are manufactured under Good Manufacturing Practices conditions and quality controlled using stringent standards to ensure product quality and consistency. SER-301 and SER-155 are formulated from pure fermented bacterial strains, ensuring that only therapeutically necessary species are included.

Our latest

Recurrent C. diff study now enrolling
An open-label study of SER-109


Target enrollment achieved in Phase 2b SER-287 study
Topline clinical data expected in mid-2021