A Study of SER-155 to Treat Diarrhea in People on Immunotherapy

Inclusion Criteria: * Age \> 18 years * Receipt of ICI (single-agent or combination) within the 180 days preceding screening. Concurrent treatment with cytotoxic chemotherapy or other tumor-directed agents is permitted. * Grade 2 - 3 diarrhea (i.e., increase of at least 4 bowel movements a day above baseline during the screening window), deemed by the treating provider as likely related to ICI therapy, with or without concomitant symptoms of grade 1 - 2 colitis (e.g. abdominal pain, bloody or mucoid stools) * Able to swallow oral medication * Individuals of childbearing potential willing to use a highly effective method of contraception (failure rate of \<1% per year when used consistently and correctly) for 30 days after the last dose of SER-155. * Willing to provide written informed consent, comply with the protocol, and understand the potential risks and benefits of study enrollment and treatment. Exclusion Criteria: * Active GI infection, including untreated viral, bacterial or fungal cause(s) of diarrhea. * Received immunosuppressive therapies for suspected or confirmed irEC, including systemic corticosteroids (either oral or intravenous) and/or infliximab, vedolizumab or ustekinumab * Grade 3 colitis symptoms, i.e. severe abdominal pain or peritoneal signs * Admitted to the hospital for irEC * Prednisone (or steroid equivalent) dose \> 10 mg a day for a non-GI irAE at time of screening * Pre-existing inflammatory bowel disease (e.g. Crohn's disease or ulcerative colitis) or microscopic colitis * Pregnant or lactating women * Any condition that requires ongoing prophylactic or therapeutic antibacterial antibiotics * Severe neutropenia, as defined by an absolute neutrophil count (ANC) \< 500 cells/mm\^3, at time of screening * Treatment with investigational medications used for diarrhea/colitis treatment and microbiome therapeutics within 30 days prior to enrollment * Known allergy or intolerance to oral vancomycin * Unable to comply with the protocol requirements * Any condition that in the opinion of the investigator may increase the risk of study participation and/or may interfere with the interpretation of study results