A Multiple Dose Study to Evaluate Safety, Tolerability, PK, and Efficacy of SER-155 in Adults Undergoing HSCT

Inclusion Criteria: * Male and female subjects ≥ 18 years of age undergoing HSCT. * Planning to undergo allogeneic hematopoietic stem cell transplantation from a human leukocyte antigen matched related donor, haploidentical related donor, HLA-matched unrelated donor, or HLA 1-antigen mismatched unrelated or related donor, with either bone marrow or peripheral blood stem cells as a graft source, and with any conditioning regimen Exclusion Criteria: * Severe colitis of any etiology or active/currently-treated inflammatory bowel disease (IBD) or total colectomy. * Evidence of relapse or progression of hematologic malignancy (minimal residual disease is allowed). * Transplant using umbilical cord blood or ex vivo T-cell-depleted HSCT * Receipt of chimeric antigen receptor T-cell (CAR-T) therapy. * Received a fecal microbiota transplant (FMT) or any live microbial therapeutic within 3 months prior to Screening. * Known allergy or intolerance to oral vancomycin. * Concomitant participation or participation within 14 days or 5 half-lives of another investigational unapproved treatment, whichever is longer.