The study will involve administering the study drug as a single dose of study drug or placebo. This study is designed to demonstrate the superiority of the experimental drug versus placebo in adult patients with recurrent CDI.

ECOSPOR is a Phase 2, multicenter, randomized, double blind, placebo controlled clinical study with 2 treatment arms (SER-109 or placebo). Patients who have diarrhea and a positive C. difficile test result on a stool sample, and who have responded to standard of care antibiotic treatment will receive study drug on Day 1.

Those patients who experience a recurrence of CDI up to 8 weeks after SER 109 or placebo treatment will be offered an opportunity to enroll in an open label SER 109 extension study (Study SERES 005).

Inclusion Criteria:

1. Signed informed consent, indicating that the patient understands the purpose of and procedures required for the study. Patients who are unable to provide informed consent will not be included in the study.
2. Male or female patients ≥ 18 years.
3. ≥ 3 episodes of CDI within the previous 9 months, inclusive of the current episode with documentation of ≥ 2 episodes.

Exclusion Criteria:

1. Female patients 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. Active irritable bowel syndrome with diarrhea within the previous 12 months.
4. Major gastrointestinal surgery (eg, 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.
5. History of inflammatory bowel disease (ulcerative colitis, Crohn's disease, microscopic colitis) with diarrhea believed to be caused by active inflammatory bowel disease in the past 24 months.
6. Admitted to or expected to be admitted to an acute care facility or intensive care unit for medical reasons (not just boarding). Patients discharged from an acute care facility before Day 1 or residing in nursing homes or rehabilitation facilities may be enrolled.
7. Concurrent intensive induction chemotherapy, radiotherapy, or biologic treatment for active malignancy (patients on maintenance chemotherapy may only be enrolled after consultation with medical monitor).