Current roles of specific bacteria in the pathogenesis of inflammatory bowel disease

The relevance of alterations in gut microbiota in the pathogenesis of inflammatory bowel disease (IBD) remains unclear. Currently there is conflicting evidence with regards to the roles of specific bacterial species. Escherichia coli (particularly the adherent invasive strain) are more prevalent in those with IBD and are associated with higher risk of IBD. However, the organisms are also present in healthy individuals and colonisation does not correlate with the degree of inflammation in IBD. Campylobacter concisus is more prevalent in those with IBD and higher levels of C. concisus specific IgG antibodies are found in the serum of those with IBD compared to healthy controls. Further, C. concisus has immunogenic properties that stimulate an antibody response suggesting the bacteria might trigger or exacerbate disease. Conversely most mycobacteria are unlikely to be causative as they are not present in microbial stool cultures early in disease. In various studies, Mycobacterium avium paratuberculosis has been detected both more frequently and not at all in individuals with Crohn’s disease. Similar conflict exists with respect to Yersinia enterocolitica, Bacteroides vulgatus and Helicobacter hepaticus, which are also more prevalent in IBD. However, these organisms appear more likely to contribute to disease persistence than initial disease development. This review aims to summarise the current understanding of key bacterial species implicated in the pathogenesis of IBD.