Safety, Dosage, and Microbiome Engraftment Study of MGT-006 for Ulcerative Colitis (ROOT Study)

Inclusion Criteria: 1. Patients who understand the contents of this clinical study, sign a written informed consent form, and possess the willingness and capability to participate in all medical examinations, evaluations, and follow-ups related to the study. 2. Patients aged 18 to 75 years at the time of informed consent 3. Patients who have been diagnosed with UC through endoscopic and histological examination for at least 12 weeks at the time of final registration 4. Patients with a modified Mayo Score of 4 to 7 at the time of final registration Exclusion Criteria: 1. Patients with any of the following medical history or complications, or those undergoing concomitant therapy, which the investigator (sub-investigator) to be serious or clinically significant 2. Patients with past or concurrent malignant tumors; however, cured basal cell carcinoma of the skin is allowed for registration. In addition, breast cancer in remission for \>= 10 years without recurrence is allowed for registration, and other malignant tumors in remission for \>= 5 years are allowed for registration if no therapeutic intervention is required and they are in stable condition, as determined by the investigator (sub-investigator). 3. Patients with pancolitis of \> 8 years' duration or left-sided colitis of \> 8 years' duration who have not undergone a surveillance flexible sigmoidoscopy or total colonoscopy (performed according to local standard of care) within 12 months prior to screening to rule out colorectal dysplasia/cancer. 4. Patients with suspected or confirmed colorectal cancer or high-grade dysplasia. 5. Patients with any of the following abnormal values in clinical laboratory tests performed within 4 weeks prior to final registration in this clinical study: * Patients with a serum creatinine level of \>= 2.0 mg/dL or a BUN level of \>= 25 mg/dL * Patients with a total bilirubin level of \>= 3.0 mg/dL, or an AST (GOT) or ALT (GPT) level of \>= 100 IU/L 6. Patients with any of the following will be excluded: * HBsAg positive * HCV antibody positive * HIV-1/2 Ag/Ab screening-positive 7. Patients currently receiving inpatient treatment for UC 8. Patients suspected to have concurrent active intestinal obstruction or infectious enteritis 9. Patients with Clostridioides difficile infection (C. difficile infection) (patients in whom the presence or absence of infection is confirmed by NAAT and the result is positive) 10. Patients with a history of hypersensitivity or allergy to excipients in MGT-006 11. Patients with any of the following conditions that affect the lower gastrointestinal function 12. Patients who have failed or been intolerant to \>= 3 advanced-therapy mechanistic classes for ulcerative colitis (e.g., anti-TNF, anti-integrin, anti-IL-12/23, JAK inhibitor, S1P modulator) 13. Patients who have used another investigational product within 12 weeks prior to informed consent 14. Patients with serious allergies or contraindications to any drug used in this study 15. (Females only) Patients in whom the result of a pregnancy test performed within 4 weeks prior to final registration is positive (if a urine hCG test is positive, a negative serum pregnancy test result should be confirmed) 16. (Females only) Patients who are pregnant or breastfeeding, or females of childbearing potential unable to use appropriate contraceptive methods from the date of informed consent to 12 weeks post-treatment. (e.g., intrauterine device, diaphragm, partner's use of condom) 17. (Males only) Patients who are males of reproductive potential (including partners of childbearing potential) unable to use at least one effective contraceptive method from the time of informed consent until 12 weeks after the end of investigational drug administration 18. Patients judged by the investigator (sub-investigator) to be inappropriate as subjects of this study 19. (Phase 2a only) Participants who have previously received MGT-006