The study will involve administering a single dose of investigational drug or placebo in ascending dose cohorts. This study is designed to evaluate the safety and tolerability of investigational drug as well as the efficacy of investigational drug versus placebo in adults with primary (first episode) Clostridium difficile infection (CDI).

SER-262-001 is a Phase 1b, randomized, double blind, placebo-controlled, ascending single dose clinical study with 2 treatment arms (SER-262 or placebo) in up to 8 dose cohorts. Patients who have been diagnosed with their first (primary) episode of CDI, defined as diarrhea (≥ 3 unformed stools per day for 2 consecutive days), a positive C. difficile stool test and who have responded to standard-of-care antibiotic will receive investigational drug or placebo on Day 1.

Inclusion Criteria:

1. Signed informed consent, indicating that the subject understands the purpose of and procedures required for the study.
2. Male or female subjects ≥ 18 years.
3. A primary (first) episode of CDI with documentation of the episode including CDI date, test results, antibiotic treatment (including start and stop dates), and response to treatment

Exclusion Criteria:

1. Females 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 12 months.
6. Estimated glomerular filtration rate (eGFR) < 60 ml/min/1.73m2
7. Admitted to or expected to be admitted to an intensive care unit for medical reasons (not just boarding). Patients discharged from an intensive care unit before Day 1 may be enrolled.
8. Concurrent intensive induction chemotherapy, radiotherapy, or biologic treatment for active malignancy (patients on maintenance chemotherapy may only be enrolled after consultation with medical monitor).
9. Absolute neutrophil count < 500 cells/mm3