Patients and Physicians

Seres announced in August 2020 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.

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 are being evaluated for their ability to 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 3 ECOSPOR III study for the prevention 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.

Please see complete clinical trial results for the ECOSPOR III study and ECOSPOR IV study.

To learn more about the Expanded Access Program, click here.


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

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 takes additional steps to inactivate potential pathogens, which represents critical intervention to increase quality assurance and to help minimize 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.