Inclusion Criteria for patients:  - Patient aged 8 to 17 years old Ulcerative colitis (UC), whatever the extent, except isolated proctitis (<5 cm), diagnosed for more than 3 months according to the usual clinical, biological and endoscopic criteria Moderate active UC defined by a PUCAI score > 35 and responding to corticosteroid treatment with a PUCAI score <10 at enrollment Treatment of UC (5-ASA, immunosuppressants, biotherapies) stable for more than 3 months Patient able to swallow test capsules  For girls of childbearing age:  To have a negative blood (or urine) pregnancy test To agree to use a reliable contraceptive method from visit 1 until the end of the research Patient with health insurance Informed written consent form signed by both parents or by the person (s) with parental authority  Inclusion Criteria for healthy volunteers donors:  Aged between 20 and 50 years Body mass index (BMI) between 17 kg / m² and 27 kg / m² Usually regular transit defined as at least one stool per day, usually issued in the morning  For women of childbearing age:  o have a negative pregnancy (or urine) blood test  Subject with health insurance Informed Written consent  Exclusion Criteria for patients:  isolated proctitis (<5 cm) severe colitis defined by a PUCAI score> 65 Being on enteral nutrition Have received antibiotic or antifungal treatment in the 4 weeks prior to enrollment Having a Clostridioides difficile infection in the 4 weeks prior to enrollment; Being pregnant or breastfeeding, or have a positive pregnancy test; Have a contraindication to colonoscopy or general anaesthesia  Exclusion Criteria for healthy volunteers donors:  - For details, please see protocol