the etiology of Inflammatory Bowel Diseases (IBD) is closely associated with the gut microbiome. The results of previous studies on the effectiveness of antibiotics and fecal macrobiota transplantation (FMT) are contradicting.

Aims: to evaluate the effectiveness of wide-spectrum antibiotic regimens in acute severe colitis in an addition to standard corticosteroid therapy (UC and isolated "UC-like" Crohn's colitis). The secondary aim is to assess the outcome of FMT in those not responding to five days of therapy (in either arm). As an exploratory aim, any IBD patient with a resistant disease to at least two immunosuppressive medications, may be treated with either interventions.

Inclusion Criteria:

• Children over the age the 2 years and adults of all ages with established diagnosis of UC using standard criteria (26, 27).
• Admission for IV steroid therapy
• PUCAI of at least 65 points at admission (i.e. severe attack)
• PUCAI>45 at enrollment
• Ability to swallow antibiotics (pills or syrup)

Exclusion Criteria:

• Change in dose or intervals of anti-TNF within the past 2 months prior to admission.
• Disease confined to the rectum (Proctitis).
• Antibiotic use in the past 4 weeks.
• Any known erosive inflammation anywhere in the small bowel or esophagus.
• Any proven infection such as positive stool culture, parasite or C. difficile, urinary tract infection, cellulitis, abscess, pneumonia, line-infections etc.
• Fever >38.5, or >38.0c thought to be unrelated to the inflammatory process of active UC.
• The probable need for second line medical therapy (infliximab, cyclosporine, tacrolimus) or colectomy within 5 days of enrollment, as judged by the caring physician.
• Known allergy to more than one antibiotic regimen from the list below.
• Pregnancy.