A Phase 1b Trial to Evaluate the Safety of MB310 in Patients With Active, Mild-to-Moderate Ulcerative Colitis

Inclusion Criteria: The main inclusion criteria include but are not limited to the following: * Must be aged 18 to 70 years, inclusive, at the time of signing informed consent; * Must have newly diagnosed or a history of recurrent UC based on clinical, endoscopic, and histological assessments; * Must have active, mild-to-moderate UC as defined by the Modified Mayo Score (MMS) of ≥4 and ≤7, and an Endoscopic Subscore of ≤2 in the most affected area proximally ≥15 cm from anal verge; * Male patients, and female patients of childbearing potential who are at risk of pregnancy, must agree to use a highly effective method of birth control * Female patients must not be pregnant or breastfeeding; * Male patients must agree to abstain from sperm donation; * Must be able to understand and comply with the Protocol requirements; and * Must be willing and able to provide written informed consent at Screening (Visit 1). Exclusion Criteria: The main exclusion criteria include but are not limited to the following: * Disease limited to proctitis \<15 cm from anal verge; * Short bowel or malabsorption syndromes; * Prior intestinal or colon resection surgery (with exception of cholecystectomy or appendectomy); * Severe/fulminant UC; * Other forms of inflammatory bowel disease including diagnostic uncertainty by the Investigator, or functional gastrointestinal disorders; * Positive stool test for parasites, bacterial pathogens, or Clostridium difficile; * Use of any of the following treatments: * Oral 5-ASA products at a dose \>3.0 g per day (unless the use is currently stable and anticipated to remain stable during the study); * Aspirin or other nonsteroidal anti-inflammatory drugs (except aspirin for cardioprotective reasons at a dose of ≤325 mg per day); * Loperamide and other antidiarrheal agents or probiotics; * Antibiotics or other antibacterial treatment (unless an ophthalmic or otic antibiotic preparation, or a topical antibiotic for skin infection); * Faecal Matter Transplant (FMT) or administration of Vowst®, Rebiotix®, or other Live Biotherapeutic Product (LBP); * Intravenous or intramuscular corticosteroids; * Oral corticosteroids \>10 mg prednisolone or equivalent per day; * Any drugs formulated for rectal administration and/or interventions; * Immunomodulating or immunosuppressing drugs (unless the use is currently stable and anticipated to remain stable during the study); * Biologics, ozanimod, etrasimod, tofacitinib, filgotinib, or upadacitinib; or * Proton pump inhibitors (PPIs) or H2 blockers. * Patients whose disease has not responded to or lost response to 2 or more advanced therapies (biologics or small molecules); * Significant liver impairment; * Concurrent primary sclerosing cholangitis; * Clinically significant hematological function abnormalities; * Known hypersensitivity, intolerance, or contraindication to oral vancomycin, MB310, and/or any excipients; * History of, or known malignancy (unless adequately treated (i.e., cured) basal cell carcinoma or squamous cell carcinoma of the skin, or cervical intraepithelial neoplasia or carcinoma in situ of the cervix with no evidence of recurrence within 5 years prior to Screening); * Any infectious disease (HIV is allowed where certain protocol-specified criteria are met); * Significant cardiovascular condition; * Involvement in another clinical study (unless observational) within 4 weeks of Screening from the last dose of study drug or 5 half-lives, whichever is longer; or * Any other clinically relevant or poorly controlled, unstable condition that would confound study endpoints or adversely affect patient safety or compliance.