Inclusion Criteria:  Of any gender Over 18 years of age Have a previous diagnosis of ulcerative colitis Have had IPAA surgery, and Have been diagnosed with pouchitis Be able to understand and complete study procedures as determined by the investigator Be able to speak either Danish or English Be able to comply with study procedures for the length of the study Use a highly effective contraception method for the duration of the trial (until day 30 in Phase I and until day 37 in Phase II), such as implants, injectables, oral contraceptives, IUD (intrauterine device), sexual abstinence or vasectomized partner.  Exclusion Criteria:  Patients with a previous allergic reaction to GM-CSF, metronidazole or fosfomycin Patients who are currently under antibiotic treatment or have received antibiotic treatment within the past 30 days Patients currently pregnant or breastfeeding Patients with ASA IV classification (American Society of Anesthesiologists physical status classification) Patients with severe pulmonary disease Patients with autoimmune thrombocytopenia Patients with severe renal impairment (eGFR < 40 ml/min) Patients with alcohol use disorder or history of drug abuse Patients currently in treatment for any malignant or hematological disease Patients with a previous history of cancer will be excluded from the study (except for patients with well-treated and stabile cancer after a control period of more than two years). Patients with anticipated compliance problems as determined by the investigator