Some of the statements in this presentation constitute "forward looking statements" under the Private Securities Litigation Reform Act of 1995, including, but not limited to, our development plans, the promise and potent

Seres Therapeutics Overview November

Some of the statements in this

presentation constitute "forward looking statements" under the Private Securities Litigation Reform Act of 1995, including, but not limited to, our development plans, the promise and potential impact of any of our microbiome

therapeutics, the ability of our clinical trials to support approval, the timing of clinical studies, the timing and ultimate results of the SER-109 safety data, the size of the market for SER-109, the sufficiency of cash to fund operations, and the

potential benefits of Seres' collaborations. Such statements are subject to important factors, risks and uncertainties, such as those discussed under the caption "Risk Factors" in the Company's Quarterly Report on Form 10-Q filed on

November 9, 2020, and its other filings with the SEC, that may cause actual results to differ materially from those expressed or implied by such forward looking statements. Any forward looking statements included herein represent our views as of

today only. We may update these statements, but we disclaim any obligation to do so. Forward looking statements

SER-109 Phase 3 success highlights that

the time for microbiome therapeutics is now

Formulated for oral delivery using

current Good Manufacturing Practices (cGMP) Consortia capture breadth of biological & functional diversity Mechanisms includes microbial engraftment in GI tract to restructure the microbiome Designed to target inflammatory & immunological

disease pathways simultaneously Ecobiotic microbiome therapeutics are encapsulated consortia of commensal bacteria with specific pharmacologic properties Seres is developing a novel drug modality that modulates the gut microbiome

End-to-End GMP Manufacturing Lead

Optimization & Bioprocess Hit-to-Lead Identification Clinical sample biorepository Microbiome Biomarker Discovery Consortia Design Pharmacological Properties Validation Oral formulation Proprietary genomic & metabolomic analytics World-class

collaborations Broad strain library & culturing know-how Genomic & host function screening In-silico drug design for functional targets Ex vivo & in vivo disease modeling Fermentation & formulation optimization platforms

Donor-derived & multi-strain fermentation Anaerobic, spore & lyophilized technologies Late clinical stage drug release assays Disease Target Identification Industry-leading, in-house research engine for drug discovery, development &

1. Collaboration with Nestl

Health Science, announced Jan. 11, 2016, regarding C. difficile and IBD programs for markets outside of North America 2. Collaboration with University of Texas MD Anderson Cancer Center and the Parker Institute for Cancer Immunotherapy, announced

Nov. 14, 2017, regarding evaluation of microbiome therapies to improve the outcomes of cancer patients treated with immunotherapy. The Parker Institute is the IND application holder for SER-401. 3. Collaboration with AstraZeneca, announced Mar. 11,

2019, regarding advancing mechanistic understanding of the microbiome in augmenting the efficacy of cancer immunotherapy, including potential synergy with AstraZeneca compounds. Broad opportunities for microbiome therapeutics Collaborators

Preclinical Phase 1b Phase 2b Phase 3 SER-109 Recurrent C. difficile Phase 3 SER-287 Ulcerative colitis Phase 2b SER-401 Metastatic melanoma in combination with anti-PD-1 MAb Phase 1b SER-301 Ulcerative colitis (Rationally-designed, fermented)

Improve response to check-point therapies; potential synergies with AZ pipeline Immuno-Oncology Infectious Disease Inflammatory Oncology SER-155 Infection, Bacteremia & GvHD in HSCT for cancer (Rationally-designed, fermented)

C. difficile Infection Overview and

SER-109 Phase 3 study

August 2020: Positive ECOSPOR III Phase

3 study read-out Primary endpoint: C. diff. recurrence, at up to 8 weeks SER-109 (n = 90) Multiply recurrent C. difficile patients (n=182) All subjects treated with standard of care antibiotics Placebo (n = 92) 0 weeks 8 weeks 24 weeks Safety

follow-up to 24 weeks Toxin testing to ensure inclusion of subjects with active rCDI, and for accuracy of endpoint Substantially higher dose vs. Phase 2 designed to result in greater and earlier microbiome restoration Placebo arm to provide

invaluable safety and efficacy data that cannot be obtained in open-label trials

Topline SER-109 Phase 3 study efficacy

results Time point SER-109 (N =90) Placebo (N =92) RR (95%CI) p-Value (p1/p2) n (%) n (%) Week 8 10 (11.1) 38 (41.3) 0.27 (0.15, 0.51) <0.001 / <0.001 Highly statistically significant 30.2% absolute reduction in the rate of CDI recurrence

compared to placebo Number needed to treat = approximately 3 Primary efficacy endpoint results: Sustained clinical response rate (i.e., percentage of patients who remain free of CDI at 8 weeks): SER-109 was effective in 88.9% of SER-109 subjects vs.

58.7% of subjects in the placebo arm Results were statistically significant in both age stratified subgroups: 18-64 years old, or 65 and over

Favorable safety profile observed in

Phase 3 SER-109 was well tolerated, with no treatment-related serious adverse events (SAEs) observed in the active arm, and an adverse event profile similar to placebo Overall incidence of patients who experienced AEs during the eight-week study

period was similar between SER-109 and placebo arms Most commonly observed treatment-related AEs were flatulence, abdominal distention and abdominal pain, which were generally mild to moderate in nature, and these were observed at a similar rate in

both the SER-109 and placebo arms

Substantial recurrent C. difficile

infection market opportunity Infectious disease caused by toxin-producing bacteria, resulting in diarrhea, abdominal pain, fever and nausea Leading cause of hospital-acquired infection in the U.S. ~ 453K cases of primary CDI within the U.S. each

year ~ 170K episodes per year (100K episodes of first recurrence; ~ 73K episodes of 2+ recurrences) Estimated ~ $5B in healthcare burden each year Source: * Desai et al., Epidemiological and economic burden of Clostridium difficile in the United

States: estimates from a modeling approach, BMC Infectious Diseases (2016) 16:303; Guh AY et al. NEJM 2020 25% of primary C. difficile recur Over 20,000 deaths per year Potential broad FDA label covering rCDI patients

FMT safety concerns highlight the

need for improved, FDA-approved treatment options for C. difficile infection In contrast to FMT, SER-109 is comprised of a highly purified consortia of spore-based bacteria manufactured under GMP conditions to ensure product quality and consistency

Unique manufacturing process to inactivate potential pathogens Process inactivates many emerging potential pathogens where diagnostic assays may not yet be widely available, such as SARS-CoV-2 Since July 2020, the largest U.S. provider of FMT has

quarantined supply and halted shipments

SER-109 open label study enrollment

ongoing to support expanded safety database Seres is currently enrolling recurrent CDI patients in an open label study to expand the SER-109 safety database, supporting a Biologics License Application (BLA) to the U.S. Food and Drug Administration

(FDA) In November 2020, FDA indicated that the SER-109 safety database should include at least 300 treated subjects with 24 weeks of follow-up, 30 days subsequent to the BLA filing With support from the FDA, Seres is including patients with a first

recurrence of CDI, in addition to those with multiple recurrent CDI, in the open label study Seres is working toward activating approximately 130 clinical sites in the U.S. and Canada to fulfill the safety database obligation, and to enable a BLA

filing, as rapidly as possible

Amplifying efforts for market

preparation and launch Medical communications strategy KOL mapping Develop and deploy payer value proposition Scaling Market Education Efforts Enhancing Understanding of Commercial Opportunity Deeper patient journey analysis Pricing analysis

Customer segmentation Identify options for go-to-market model Building Infrastructure to Launch Scale Medical Affairs organization and deploy MSL team Hire key commercial leadership roles Key external strategic partners on board

Strong clinical & scientific

data Dramatic reduction in CDI recurrence rate Spore-forming Firmicute bacteria prevent C. difficile germination and growth SER-109: Investigational, spore-based therapeutic designed to break the cycle of recurrent C. difficile infection Oral

formulation Spores are resistant to gastric acid, facilitating oral delivery to gastrointestinal tract Favorable safety profile Favorable tolerability & safety profile with no imbalance in adverse event Spore

purification mitigates risk of transmission of known and unknown infectious agents SER-109 Oral formulation of Firmicute bacteria spores FDA regulatory designations Breakthrough designation Orphan drug status

SER-287 and Ulcerative

Serious chronic condition

characterized by inflammation of the colon and rectum resulting in abdominal pain, bowel urgency and diarrhea Significant need for improved therapies - Many drugs are immunosuppressive, limiting use to more severe patients ~700K in the United States

Only ~1/3 achieve remission Ulcerative colitis overview

Steroids Thiopurines / MTX Anti-TNFs

JAK Inhibitors Anti IL12/23 Microbiome Anti-Integrins S1P1 Agonists Gut Lumen Lamina Propria Blood vessel Gut Epithelium Microbiome therapeutics may drive therapeutic benefit May address drivers of inflammation, barrier integrity, innate immune

activation, and adaptive immune education and cell trafficking Effector molecules may include short chain fatty acids, secondary bile acids, tryptophan metabolites, and TLR ligands Microbial consortia can likely target multiple pathways

simultaneously Opportunity to develop both first-line and combination therapies The dysbiotic microbiome may be a trigger of inflammation in ulcerative colitis

Published study regarding microbiota

transplantation provided clinical proof-of-concept in ulcerative colitis Note: SER-287 was not involved in this study

DAILY Primary Objectives Safety and

tolerability Change in composition of intestinal microbiome at 8 weeks (n=11) (n=15) (n=15) (n=17) Placebo pre-treatment for 6 days 58 mild-to-moderate ulcerative colitis patients Placebo once daily for 8 weeks Vancomycin pre-treatment for 6 days

SER-287 once daily for 8 weeks Placebo pre-treatment for 6 days SER-287 once weekly for 8 weeks Vancomycin pre-treatment for 6 days SER-287 once weekly for 8 weeks WEEKLY SER-287 Phase 1b ulcerative colitis study

Remission = Total Modified Mayo

score 2 AND endoscopic subscore 1 Note: Missing data treated as failure; statistical significance not found in SER-287 weekly arms Phase 1b study results - Statistically significant clinical remission improvement observed in

Vanco/SER-287 daily treatment arm Placebo pretreatment / Placebo Vancomycin pretreatment / SER-287 daily Placebo pretreatment / SER-287 weekly Vancomycin pretreatment / SER-287 weekly p = 0.0237 (0/11) (6/15) (2/15) (3/17)

Illustrative endoscopy improvement

- Vanco/SER-287 daily treatment Pre-treatment endoscopy showing the sigmoid colon with spontaneous bleeding and ulceration Post-treatment day 64 endoscopy

SER-287 Phase 1b safety results show

safety profile comparable to placebo SER-287 daily arm demonstrated a similar safety profile to placebo No serious drug-related adverse events Reduced gastrointestinal adverse events provide an independent assessment of efficacy as the GI adverse

events likely reflect ulcerative colitis disease activity SER-287 daily arm GI AEs: 2/15 (13.3%) vs. placebo arm: 5/11 (45.5%)

Phase 1b study results -

SER-287 bacteria engrafted in subjects and was durable to four weeks after dosing Significant engraftment observed starting one week post-dosing Engraftment is significantly higher in arms with vancomycin pre-conditioning Engraftment in vancomycin

arms is dose-dependent; significantly greater in daily dosing arm (arm with greatest efficacy) Henn et al. (in review)

SER-287 Phase 1b study results

Ongoing SER-287 ECO-RESET Phase 2b

study in patients with mild-to-moderate active ulcerative colitis 10-week induction period Pbo pre-treat Mild to moderate UC patients with active disease N=201 Vanco pre-treat Vanco pre-treat Placebo SER-287 daily high dose SER-287 daily high dose

followed by step down dose 26-week exploratory maintenance follow-up FDA Fast Track designation FDA feedback: Phase 2b study results, in conjunction with data from a second pivotal study, could support BLA submission Over 75% enrolled (as of Nov. 9,

2020) Primary endpoint: Clinical remission

Earlier stage development programs:

SER-401, SER-301, SER-155

SER-301: Next-generation, rationally

designed fermented microbiome therapeutic candidate for ulcerative colitis Reduces induction of pro-inflammatory activity Improves epithelial barrier integrity & TNF- driven inflammation in IECs Modulates UC-relevant anti-inflammatory,

innate & adaptive immune pathways

SER-301: Ongoing Phase 1b in adults

with mild-to-moderate ulcerative colitis Screening Induction Treatment (daily dosing) Safety Period Pre- Cond. 10 weeks 16 weeks * * endoscopy, pre- and post-treatment 6d Cohort 1, Open Label (15 subjects, SER-301) SER-301 after pre-conditioning

Cohort 2, Placebo-controlled (50 subjects, 2:3 Placebo to SER-301) Placebo after placebo SER-301 after pre-conditioning Study Objectives Primary endpoint = safety and tolerability Assess drug activity via changes in biomarkers of clinical remission

in patients treated with SER-301 Secondary endpoints = engraftment and measures of clinical remission First patient dosed in November 2020

Gopalakrishnan et al, Science 2017

SER-401 composition driven by bacteria consistent with responder profile All spore formers that leverage deep Seres expertise in the biology and manufacturing of these organisms SER-401: Immuno-oncology - Microbiome signature in melanoma patient

responder to anti-PD-1

Patients with metastatic melanoma

treated with anti-PD-1 (nivolumab) SER-401; biologically sourced product to match microbiome signature of anti-PD-1 responders (n=20) Placebo (n=10) Day -14 Day -7 Day +7 Day 0 Day +14 Day +28 Day +56 Biospecimens: SER-401 / Placebo Day +84 Blood

Stool Biopsy SER-401: Ongoing Phase 1b study Study Objectives Primary endpoint = safety and tolerability Assess drug activity via microbiome engraftment and its association with biomarkers of clinical response in tumor biopsies in patients treated