A Study to Evaluate the Safety, Tolerability and Microbiome Dynamics of SER-287 in Subjects With Mild-to-Moderate Ulcerative Colitis

Inclusion Criteria: * Ulcerative colitis diagnosed by routine clinical, radiographic, endoscopic and pathologic criteria (preferably confirmed by colonoscopy and pathology records within last 2 years or if unavailable, will need approval by medical monitor) Active mild-moderate UC as determined by sigmoidoscopy within approximately 3 days of randomization to study Exclusion Criteria: 1. Fever \> 38.3°C 2. Known or suspected toxic megacolon and/or known small bowel ileus 3. Known history of Crohn's disease 4. Subjects with serum albumin \<2.5 g/dL at baseline 5. CMV polymerase chain reaction (PCR) positive from blood plasma at screening 6. Known stool studies positive for ova and/or parasites or stool culture within the 30 days before enrollment 7. Subjects on cyclosporine or triple immunosuppression, Triple immunosuppression will include any three of the following classes of drugs taken in combination: steroids (i.e., prednisone/budesonide/budesonide MMX), immunosuppressant (i.e., methotrexate/azathioprine/6-mercaptopurine), and/or other immunosuppressant (i.e., tacrolimus, cellcept). 8. Biologic medication (infliximab/ adalimumab/ golimumab/ certolizumab/vedolizumab/ustekinumab/natalizumab) use within 3 months prior to screening 9. Known active malignancy except for basal cell skin cancer, squamous cell skin cancer 10. Subjects with previous colectomy, ostomy, J-pouch, or previous intestinal surgery (excluding cholecystectomy, appendectomy) 11. Subjects with known history of celiac disease or gluten enteropathy 12. Subjects with Clostridium difficile positive stool at Screening Visit 13. Antibiotic use within the prior 1 month before randomization 14. Expected to receive antibiotics within 8 weeks of signing the Informed Consent Form (ICF) (i.e., for planned/anticipated procedure) 15. Received an investigational drug within 1 month before study entry 16. Received an investigational antibody or vaccine within 3 months before study entry 17. Previously enrolled in a SER-109/SER-287 study 18. Received an FMT within the last 6 months 19. Subjects with anatomic or medical contraindications to flexible sigmoidoscopy, including but not necessarily limited to toxic megacolon, gastrointestinal (GI) fistulas, immediate post-operative status from abdominal surgery, severe coagulopathy, large or symptomatic abdominal aortic aneurysm, or any subject where study physician deems subject at significant risk of complications of flexible sigmoidoscopy 20. Unable to stop steroid enemas or suppositories or mesalamine enemas or suppositories before screening visit 21. Unable to stop opiate treatment unless on a stable dose and no increase in dose planned for the duration of the study 22. Unable to stop probiotics before screening visit 23. Concurrent intensive induction chemotherapy, radiotherapy, or biologic treatment for active malignancy (subjects on maintenance chemotherapy may only be enrolled after consultation with medical monitor) 24. Known allergy or intolerance to oral vancomycin \-