Efficacy and Mechanism of Fecal Microbiota Transplantation of the Bai Ethnicity in the Treatment of UC

Inclusion Criteria: 1. Age between 14 and 79 years (inclusive), any gender. 2. Diagnosed with ulcerative colitis (UC) per established clinical, endoscopic, and histological standards, with a disease duration of over 3 months. 3. Active mild-to-moderate UC, defined by a Mayo score of 4-10, including an endoscopic score ≥1 and a physician's global assessment score ≤2. 4. Stable baseline medication consisting of 5-aminosalicylic acid (mesalamine). 5. Signed written informed consent. Exclusion Criteria: 1. Participants unable to provide informed consent, answer questionnaires, or supply samples. 2. Pregnant women or those attempting to conceive. 3. Participants unwilling to use effective contraception throughout the study. 4. Participants deemed in remission by investigators. 5. Evidence or history of toxic megacolon. 6. Isolated rectal inflammation (\<5 cm in extent). 7. Diagnosed with Crohn's disease or indeterminate colitis. 8. Participants with perianal diseases (e.g., fistulae, anal fissures). 9. History of significant gastrointestinal surgery (e.g., colectomy) ： * Minor surgeries will be reviewed case by case. * Patients with appendectomy within 3 months will be excluded. 10. Antibiotic use within the past 4 weeks for any reason, including for UC. 11. Steroid dependence requiring \>20 mg prednisone or \>9 mg budesonide daily at enrollment. 12. Recent or anticipated usage of prohibited drugs during the study period, including: * Rectal corticosteroids within 2 weeks prior to the first FMT. * Biologics (e.g., infliximab, adalimumab, vedolizumab) within 4 weeks prior to the first FMT. * Other major immunosuppressants (e.g., calcineurin inhibitors, antitumor drugs) within 12 weeks prior to treatment. * Probiotics within 4 weeks before the first FMT. * Experimental drugs or protocols within 12 weeks before the first FMT. * Anti-tuberculosis (TB or MAC) treatment within 4 weeks before the first FMT. Permitted Medications: Participants may continue using the following medications if doses are stable within specified timeframes before the first FMT: * Oral 5-aminosalicylic acid (stable for 4 weeks). * Azathioprine and methotrexate (≥90 days of use with stable doses for 4 weeks). * Oral prednisone (≤20 mg/day, stable for 2 weeks, gradually tapered at a rate of 2.5 mg/week to discontinue by week 8). Subjects should maintain the same doses of oral 5-aminosalicylates, thiopurines, and methotrexate during the study. For oral prednisolone, the dose had to be tapered off gradually, at a rate of 2.5mg per week, so that subjects were no longer exposed to steroids until week 8. Prohibited Medications: * Rectal corticosteroids (2 weeks before and throughout the study). * Antibiotics, antifungals, antivirals, probiotics, or prebiotics (4 weeks before and throughout the study). * Biologics or calcineurin inhibitors (12 weeks before and throughout the study). Participants using prohibited medications during the study will remain enrolled, and outcomes will still be evaluated. All prohibited medication usage will be recorded.