Subjects will receive an oral dose of SER-109 in 4 capsules once daily for 3 consecutive days in Treatment Group I or matching placebo once daily for 3 consecutive days in Treatment Group II. The purpose of this study is to demonstrate the superiority of SER-109 vs placebo to reduce recurrence of CDI as determined by a toxin assay in adults up to 8 weeks after initiation of treatment.

ECOSPOR III is a Phase 3, multicenter, randomized, double-blind, placebo-controlled, parallel-group study of the safety, tolerability, and efficacy of SER-109 versus placebo in adult subjects 18 years of age or older with recurrent CDI, defined as: a history of ≥ 3 CDI episodes within 12 months, inclusive of the current episode. This study is designed to demonstrate the superiority of SER-109 versus placebo to reduce recurrence of Clostridium difficile infection (CDI) in adults who have received antibacterial drug treatment for recurrent CDI (RCDI), based on the proportion of subjects experiencing a CDI recurrence requiring antibiotic treatment up to 8 weeks after initiation of treatment. Approximately 188 subjects with a history of CDI, diarrhea and a positive C. difficile toxin test result on a stool sample, who have responded to standard-of-care (SOC) antibiotic treatment will be enrolled. Subjects will be randomly assigned, in a 1:1 ratio, to 1 of 2 treatment groups (Treatment Group I [SER-109] or Treatment Group II [Placebo]) and stratified by age (<65 years; ≥65 years), as well as antibiotic regimen for the qualifying episode (vancomycin; fidaxomicin).

Subjects will receive an oral dose of SER-109 in 4 capsules once daily for 3 consecutive days in Treatment Group I or matching placebo once daily for 3 consecutive days in Treatment Group II.

Subjects with confirmed CDI recurrence, as defined in the Protocol, up to 8 weeks after administration of SER-109 or placebo treatment, may be eligible to enroll in the open-label SER-109 extension study (Study SERES-013).

Main Inclusion Criteria:

1. Signed informed consent prior to initiation of any study-specific procedure or treatment. The subject or their legally authorized representative must be able to provide written informed consent and understand the potential risks and benefits from study enrollment and treatment.

2. Male or female subject ≥ 18 years of age.

3. A qualifying episode of CDI as defined by:

   1. ≥ 3 unformed stools per day for 2 consecutive days
   2. A positive C. difficile stool toxin assay.
   3. The requirement of CDI SOC antibiotic therapy (defined as 10 to 21 days of treatment with vancomycin [125 mg QID] and/or fidaxomicin [200 mg BID]).
   4. An adequate clinical response following SOC antibiotic therapy, defined as (<3 unformed stools in 24 hours) for 2 or more consecutive days before randomization.

Main Exclusion Criteria:

1. Female subjects who are pregnant, breastfeeding, lactating, or planning to become pregnant during the study.
2. Known or suspected toxic megacolon and/or known small bowel ileus.
3. Admitted to or expected to be admitted to an intensive care unit for medical reasons (not just boarding). Note: nursing homes, rehabilitation, assisted living centers and acute care hospitals are acceptable.
4. Absolute neutrophil count of <500 cells/ml^3
5. Major gastrointestinal surgery (e.g. significant bowel resection or diversion) within 3 months before enrollment (this does not include appendectomy or cholecystectomy), or any history of total colectomy or bariatric surgery (bariatric surgery which does not disrupt the gastrointestinal lumen, i.e., restrictive procedures such as banding, are permitted).
6. History of active inflammatory bowel disease (ulcerative colitis, Crohn's disease, microscopic colitis) with diarrhea believed to be caused by active inflammatory bowel disease in the past 3 months.
7. Concurrent intensive induction chemotherapy, radiotherapy, or biologic treatment for active malignancy (subjects on maintenance chemotherapy may only be enrolled after consultation with the study medical monitor).
8. Any history of fecal microbiota transplantation (FMT) within the previous 3 months.