I’ve talked here before about the complex ecosystem of bacteria and other microbes that exists inside each of us, and about the concern that some of our well-intentioned efforts at improving health may upset a delicate equilibrium that helps keep us healthy. Now some biologists at Cal Tech have discovered what may be some additional pieces of the puzzle.
According to a report at the PhysOrg site, disturbances to the microbial equilibrium in the gut have been linked to inflammatory bowel disease and colon cancer.
“It has been proposed that the coupled equilibrium between potentially harmful and potentially beneficial bacteria in the gut mediates health versus disease,” says Sarkis K. Mazmanian, assistant professor of biology at Caltech. “If the balance is altered,” say, by changes in diet, the effects of stress, or the use of antibiotics, “then the immune response in the intestines is also changed.”
The normal microbial population of the gut includes something like 1000 different species of bacteria. It may include some distinctly harmful (pathogenic) bacteria; it also contains a large number of beneficial bacteria (symbionts). The CalTech researchers suggest that there is a third category, which they have dubbed “pathobionts”, exemplified by an organism called Helicobacter hepaticus. This organism can live in the gut of a healthy individual for many years without causing any ill effects; however, it can cause symptoms similar to inflammatory bowel disease in individuals whose immune system is compromised.
The team found that the “tolerant” relationship in healthy individuals was maintained by the bacterium’s secretion system, “a collection of proteins the microbe uses to send chemical messages to its host.” These messages apparently work to negotiate a “cease fire” between the microbe and the immune system. When the secretion system is disrupted, the population of H. hepaticus increases sharply, and the activity of the immune system also increases, producing inflammation.
“The bacteria appear to have struck a deal with their host,” Mazmanian says. They keep their own numbers low so they don’t overwhelm the immune system, and in return, the immune system leaves them alone. “The bacteria need the secretion system to put the host in ‘don’t attack’ mode.”
The paper describing this research appears in the April 22 issue of Cell Host & Microbe, and is available online here.