It has been estimated that there are ten times as many bacterial cells as human cells in the body, with the vast majority of bacteria living in the intestine. Around 500 bacterial species are present in the normal human gut and generally provide a beneficial service, synthesizing vitamins such as folic acid, vitamin K and biotin, fermenting complex carbohydrates, and converting lactose to lactic acid. The presence of such bacterial colonies also inhibits the growth of potentially pathogenic bacteria. The microorganisms which populate the gut, termed the commensal microbiotica, are also actively involved in immune regulation and homeostasis and the composition of the microbiota has been suggested to influence susceptibility to inflammatory bowel diseases.
In recent years, IL-17-producing T-helper (Th17) cells have been recognised to be involved in a wide variety of inflammatory conditions and autoimmune diseases. A report in the journal Cell Host and Microbe shows that the small intestine provides an environment that uniquely favours differentiation of Th17 cells which are scarce elsewhere in the body. The composition of the commensal intestinal bacteria was found to have a crucial role in the differentiation of SI LP Th17 cells and in their balance with Treg cells, which also make up a large proportion of CD4+ T cells in the intestinal mucosa. Only a subset of vancomycin-sensitive bacteria were found to induce Th17 cell differentiation, suggesting that unique innate immune signaling pathways, distinct from the TLR-mediated signals that can be initiated by numerous microorganisms, are required for this process. In experiments in mice, the presence of Th17 cells in the mucosa correlated with the presence of members of the cytophaga-flavobacter-bacteroidetes (CFB) phylum, implicating these bacteria as Th17 cell inducers. This is the first report linking a defined set of gut flora to a specific immune response and could help in the development of novel treatments for inflammatory bowel disease and other diseases.