Seres Therapeutics Appoints Genomic Medicine and Microbiology Pioneer Claire M. Fraser, Ph.D., to its Board of Directors and Announces SER-155 Phase 1b Study Progress

– Renowned investigator in microbial genomics to join Seres to advance Company’s efforts to develop novel microbiome therapeutics –

– Following successful Data Safety Monitoring Board meeting in December for SER-155 Cohort 1, enrollment of Cohort 2 planned to begin in early 2023 –

Seres Therapeutics, Inc. (Nasdaq: MCRB) today announced the appointment of Claire M. Fraser, Ph.D., to its Board of Directors. Dr. Fraser will begin serving on Seres’ Board, effective immediately.

“It is an incredibly exciting time to be joining the Seres Board of Directors as the company prepares for its first potential product approval with SER-109 in recurrent C. difficile infection,” said Dr. Fraser. “In my view, the success of SER-109 bodes well for Seres’ broader pipeline of new clinical candidates using microbiome therapeutics as a novel approach for Infection Protection for medically compromised individuals and antimicrobial resistance more generally. I look forward to being a part of a company working diligently to continue to bring the promise of such microbiome therapeutics to patients.”

Dr. Fraser is the Director of The Institute for Genome Sciences, which she launched in 2007, at the University of Maryland School of Medicine. She also serves as a faculty member in the Departments of Medicine and Microbiology/Immunology. Prior to this, she held positions as President and Director of The Institute for Genomic Research; Chief of the Section on Molecular Neurobiology in the Laboratory of Physiologic and Pharmacologic Studies at the National Institute on Alcohol Abuse and Alcoholism; Senior Staff Fellow and Chief of the Unit of Receptor Regulation, Receptor Biochemistry and Molecular Biology at the Laboratory of Molecular and Cellular Neurobiology within the National Institutes of Health; and a Cancer Research Scientist at Roswell Park Memorial Institute.

She graduated from Rensselaer Polytechnic Institute in Troy, NY and received her Ph.D. in pharmacology from the State University of New York at Buffalo. She was elected as a member of the National Academy of Medicine in 2011 and recently concluded a term as President of the American Association for the Advancement of Science (AAAS).

“I look forward to collaborating with Dr. Fraser, whose work has provided important contributions that have supported the emergence of the microbiome industry for years,” said Eric Shaff, President and Chief Executive Officer of Seres. “We continue to advance our pipeline, delving deeper into our Infection Protection strategy to address unmet needs in medically compromised individuals, including those with cancer neutropenia, cirrhosis, and solid organ transplant. Dr. Fraser’s experience will serve as the perfect complement to the knowledge and development expertise we have built at Seres over the last decade.”

“Dr. Fraser brings deep expertise driving forward the science of the microbiome and we expect her to be an invaluable addition to our Board supporting the development of future microbiome medicines and the continued success of Seres,” said Stephen Berenson, Chairman of the Board of Seres and Managing Partner of Flagship Pioneering.

In addition to the focus on SER-109, Seres continues to make progress on its SER-155 clinical study. Seres recently completed enrollment of Cohort 1 in its SER-155 clinical study and, after a successful pre-planned meeting in December 2022 with the study’s Data and Safety Monitoring Board (DSMB) to review SER-155 Cohort 1 safety data, plans to begin enrollment of Cohort 2 in early 2023. In addition, the Company plans to announce initial safety and pharmacological data, including drug bacterial species engraftment from Cohort 1, in early 2023.

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 prevent further recurrences of C. difficile infection and has potential to become a first-in-class FDA-approved oral 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 Dr. Fraser’s appointment and the anticipated impact thereof, the potential approval and launch of SER-109; the potential for microbiome therapeutics to protect against infection; and other statements which 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: 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 (SEC), on November 2, 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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