The etiology of inflammatory bowel disease (IBD) is currently unknown. Although there is convincing evidence supporting a critical role for micro-organisms in the pathogenesis of IBD, the specific organisms involved remain undetermined. Mucus-associated bacteria have recently been considered as likely candidates for involvement in IBD; indeed several mucus-associated bacteria species have been shown to induce IBD-like conditions in animal models, and some of these bacteria have been detected in human intestinal tissues. Further studies are required to investigate the possible role of mucus-associated bacteria in human IBD.