Patients and Physicians

Seres has announced positive topline results in the Phase 3 ECOSPOR III study of SER-109 for recurrent C. difficile infection. These results were published in the New England Journal of Medicine in January 2022. Additionally, the FDA has accepted the SER-109 BLA for Priority Review.

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 published data for the Phase III ECOSPOR III study for the treatment of recurrent C. difficile in the New England Journal of Medicine.

Clostridioides difficile, formerly known as Clostridium difficile, or C. difficile, is one of the top 3 most urgent bacterial threats in the United States, according to the Centers for Disease Control and Prevention. It is the leading cause of hospital-acquired infection in the US 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 has completed enrollment in the SERES-013 ECOSPOR IV open-label study for SER-109.

Expanded Access Program for SER-109

Seres Therapeutics (“Seres“) believes that the best approach to enable all patients to access medicines is through established regulatory approval processes and commercial availability of that medicine. However, an expanded access program (also known as “EAP“) and sometimes called “compassionate use” program, is a potential way for a patient with an immediately life-threatening or serious disease or condition to gain access to an investigational medical product for treatment outside of clinical trials when no comparable or satisfactory alternative therapeutic options are available.

It is important to remember that investigational products have not been approved or cleared by regulatory bodies like the FDA for their specific use. Doctors and patients should consider all possible benefits and risks when seeking expanded access to an investigational product.

Seres will consider granting expanded access to this investigational drug only if the following criteria are met (along with other defined eligibility parameters for disease under study):

  • The patient has a serious or life-threatening illness with no comparable or satisfactory alternative therapies; the patient is no longer responsive to, or able to tolerate, available therapies; or the patient has a relevant medical condition, that in the opinion of the physician, makes an approved agent unsuitable for the patient.
  • The patient is ineligible for, or otherwise unable to, participate in a clinical trial.
  • The patient has a disease for which there is sufficient evidence indicating that the potential benefits of expanded access outweigh the known or anticipated risks to the patient (and such risks are not unreasonable in the context of the disease or condition to be treated).
  • The investigational drug is currently in clinical development—that is, it is currently being studied in humans. Providing the investigational drug for the requested use will not interfere with the initiation, conduct, or completion of clinical trials.

How to Apply to Seres’ Expanded Access Program:

Seres has established an EAP for our investigational agent, SER-109, for patients with recurrent C. difficile infection at sites who have participated in previous studies.

All patients who have been recently diagnosed with recurrent C. difficile infection by their treating physician and are interested in participating in the SER-109 Expanded Access program should discuss with their treating physician who can then contact and may receive a referral to one of the sites conducting the Expanded Access Program. Further information about the SER-109 EAP can also be found on

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 who have experience with treating patients in SER-109 clinical studies. If you are a physician interested in referring patients to an EAP 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

Every effort will be made to acknowledge inquiries submitted before 5PM EST, Monday through Friday, within 24 hours. Inquiries received after 5PM EST Friday will be addressed the following Monday. Similarly, inquiries received during a holiday will be addressed the following business day.

Upon receipt of a request for access to an investigational medicine, the requesting physician will receive more details about the guidelines around participation in the Seres EAP.

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


Phone: 617-945-9626, extension 304


SER-109 is still being tested and has not been approved by any regulatory authorities, including the US Food and Drug Administration (FDA).

There is no guarantee of access to an investigational medicine for any individual. Seres may revise or stop this program at any time.

This information is not intended to replace the advice of a healthcare professional and should not be considered as a recommendation. Patients should always seek medical advice before making any decisions on their treatment. Healthcare professionals should always refer to the specific labeling information approved for the patient’s country. The information on the website should not be considered as prescribing advice.

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:


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 Cohort 1 of a clinical trial of SER-301. Seres plans to continue research activities evaluating ulcerative colitis, including evaluating the potential to utilize biomarker-based patient selection and stratification for future studies.

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. Seres has also completed the Phase 2B ECO-RESET Study. See more information here.

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:

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 US 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 that can be delivered orally, while mitigating risk to patients.

While FMT is minimally processed donor stool that contains bacteria, fungi, and viruses, Seres utilizes protective processes in manufacturing to enhance safety. Whether Seres manufactures using fermentation of pure bacterial strains that are then combined to produce the therapeutic (where only therapeutic bacterial strains are present, as in 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 a 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 a 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.