Bacteria, good and bad: Host–microbiota interactions in inflammatory bowel disease

Inflammatory bowel disease (IBD), a term encompassing the conditions ulcerative colitis (UC) and Crohn's disease, affects around 1 in 300 people in the UK and causes significant morbidity, although, thankfully, little mortality1. Although UC is limited to the colon and causes mucosal ulceration, Crohn's disease can affect any part of the intestine and causes ulceration, transmural inflammation, stricturing, fistula and abscess formation. Both are relapsing and remitting diseases and, despite advances in medical treatments, including immunosuppressants and drugs which specifically block pro-inflammatory molecules (tissue necrosis factor; TNF), about 25% with UC and 50–80% with Crohn's disease will require major surgery at least once in their lives.