– Data underscore the ability of Seres’ lead therapeutic candidate to repair the microbiome –

Seres Therapeutics, Inc. (Nasdaq: MCRB), a leading microbiome therapeutics company, today announced that it will present data from its ECOSPOR III trial of SER-109 at the Digestive Disease Week^® (DDW) Annual Meeting, which is taking place May 21-24, 2022. SER-109 is an investigational oral microbiome therapeutic for the prevention of recurrent C. difficile infection (rCDI). With an oral and poster presentation, Seres will present data on how SER-109 restores the functionality of the microbiome in a rapid and durable manner, which further supports the potential of microbiome therapeutics. Seres expects to finalize a Biologics License Application (BLA) submission for SER-109 with the U.S. Food and Drug Administration (FDA) in mid-2022.

“We are eager to showcase new data that further validate the potential of our investigational microbiome therapeutic, SER-109, at the world’s largest gathering of physicians and researchers in the digestive disease field,” said Lisa von Moltke, M.D., Chief Medical Officer at Seres. “These data reinforce our strongly-held belief in the power of the microbiome, and we are hopeful of delivering the first-ever FDA-approved microbiome therapeutic, a potentially transformative treatment option to patients in need.”

Seres’ data presentations at DDW include:

• Oral Presentation: Impact of SER-109, an Investigational Microbiome Therapeutic, on Stool Fatty Acid Metabolites in a Phase 3 Randomized Trial (ECOSPOR III) for Treatment of Recurrent Clostridioides Difficile Infection (CDI); May 21, 11:15 - 11:30 AM PDT; Lead Author: Jessica Bryant, Ph.D.
• Poster Presentation: Engraftment of Investigational Microbiome Therapeutic SER-109 is Durable through 24 Weeks in a Randomized Trial (ECOSPOR III) for the Treatment of Recurrent Clostridioides Difficile Infection (rCDI); May 22, 12:30 - 1:30 PM PDT; Lead Author: Christopher Desjardins, Ph.D.

Posters and presentations will be available for 90 days on the DDW conference website.

About SER-109

SER-109 is an oral microbiome therapeutic candidate consisting of a consortium of highly purified Firmicutes spores, which normally live in a healthy microbiome. SER-109 is designed to prevent further recurrences of CDI by modulating the disrupted microbiome to a state that resists C. difficile colonization and growth. The SER-109 manufacturing purification process is designed to remove unwanted microbes, thereby reducing the risk of pathogen transmission beyond donor screening alone. The U.S. FDA has granted SER-109 Breakthrough Therapy designation and Orphan Drug designation for the treatment of rCDI.

About Seres Therapeutics

Seres Therapeutics, Inc. (Nasdaq: MCRB) is a leading microbiome therapeutics company developing a novel class of multifunctional bacterial consortia that are designed to functionally interact with host cells and tissues to treat disease. Seres’ SER-109 program achieved the first-ever positive pivotal clinical results for a targeted microbiome drug candidate and has obtained Breakthrough Therapy and Orphan Drug designations from the FDA. The SER-109 program is being advanced to reduce the recurrence of C. difficile infection and has potential to become a first-in-class FDA-approved microbiome therapeutic. Seres is evaluating SER-155 in a Phase 1b study in patients receiving allogeneic hematopoietic stem cell transplantation to reduce incidences of gastrointestinal infections, bloodstream infections and graft-versus-host disease as well as additional preclinical stage programs targeting Infection Protection in medically compromised patients. The Company is also conducting research to inform further development of microbiome therapeutics for ulcerative colitis.

For more information, please visit www.serestherapeutics.com.

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 the timing of a potential BLA approval of SER-109; the ultimate safety and efficacy profile; and the possibility of SER-109 being a first in class therapeutic.

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: we have incurred significant losses, are not currently profitable and may never become profitable; our need for additional funding; our limited operating history; the impact of the COVID-19 pandemic; our unproven approach to therapeutic intervention; the lengthy, expensive and uncertain process of clinical drug development; our reliance on third parties and collaborators to conduct our clinical trials, manufacture our product candidates and develop and commercialize our product candidates, if approved; and our ability to retain key personnel and to manage our growth. These and other important factors discussed under the caption “Risk Factors” in our Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission, or SEC, on May 4, 2022, and 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, 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.

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