The Symbiotic Microbiome in Charge of Training the Immune System

Source: Mazmanian et al (2008). A microbial symbiosis factor prevents intestinal inflammatory disease. Nature 453:620-625.

In general, reduced exposure to infectious agents throughout childhood increases the chance of allergic and auto-immune disease. Improvements in personal hygiene and the rampant use of antibiotics have direly affected our associations with our symbiotic microbiome. This paper is a very good example of such deregulations where the authors name the absence of Bacteroides fragilis as a cause for the emergence of colitis and other IBDs.

Here, the authors have shown that B. fragilis is essential for protection against colitis. Their experiments involves testing the germ-free mice grown in sterile conditions. Apparently, the presence of B. fragilis in the intestine switches the uneducated T cells (CD4⁺ CD45Rb^high) to educated T cells (CD4⁺ CD45Rb^low) that possess significant anti-inflammatory properties. B. fragilis affects the immune system through the production of PSA (polysaccharide A). ΔPSA strains lose their protective ability. PSA induces IL-10 expression in the intestine which is a potent anti-inflammatory agent.

We should utterly remember that B. fragilis is only one of the thousand symbiotic bacteria. Evolutionary interactions may have very well shaped our symbiotic bacterium as an often-forgotten organ.