Non-pathogenic bacteria colonize virtually every tissue type with exposure to the external environment; the enteric bacteria found in the gastrointestinal (GI) tract, which contains by far the largest number and species of bacteria, have long been known to provide benefits such as delivering certain nutrients and protection against pathogenic bacteria. In a series of studies, Mazmanian and colleagues have teased out the complex interplay between gut bacterial species and the host immune system. His results show, for example, that the probiotic species B. fragilis expresses a molecule known as polysaccharide A, which suppresses a specific immune pathway in the host that would otherwise generate an inflammatory response to a different bacterium, H. hepaticus. Absent this suppression, H. hepaticus triggers pathologic changes in the GI tract similar to ulcerative colitis and Crohn�s disease.

I've heard several times now that inflammation is the culprit.

But it's not so simple -- as you point out, there's complex interplay between bacteria and the host.

Keep an eye out for other articles and videos on immune disorders and inflammation, please. I'm very interested.