Harvard Medical School researchers have found that gut bacteria release molecules that reduce inflammation and protect against colitis, suggesting that intestinal bacteria actively network with the immune system to promote health.

A common gut bacterium produces a molecule called PSA, which inhibits immune-cell production of chemicals that trigger inflammation in response to infections.

The Harvard researchers inoculated mice with PSA-producing or non-producing bacteria, then gave them another bacteria that causes colitis in mice. The mice with normal (PSA producing) bacteria were protected from colitis.

The research supports the “hygiene hypothesis”: reduced exposure to infections during early childhood may increase the risk of allergic and autoimmune disease. It also implies that the human genome does not encode all the factors needed for the development of a healthy immune system, but that our health depends on critical interactions with the collective genomes of our gut flora.