Your immune system works hard everyday to protect you against the hostile external world. A sneeze on the train, an undercooked hamburger or a splinter from a park bench all incite an immune response that keeps your body healthy. It seems logical that an immune system designed to protect you against bad bugs would see the trillions of bacteria living in your gut as a problem; however, that is not the case at all.
Amazingly, a healthy intestinal flora helps support your immune defenses. In fact, animal studies show that if we didn’t have our healthy intestinal flora, our immune system would not be the same. Those trillions of critters are necessary for proper immune function – in a host of different ways.
The first major function of a healthy microbiota (intestinal flora) is physical reinforcement of your digestive tract. You see, just as your skin is, the digestive tract is an important barrier between you and the outside world. When this barrier function is intact, microbes in the outside world can’t activate a major immune response. To help protect the gut barrier, your gut cells secrete mucus to make it more difficult for microbes to attack the gut cell.
Your healthy gut microbiota assist in this defense system by nestling into the mucus and providing a further physical blockade to prevent harmful critters from attacking the gut. Even better, gut bacteria can help stimulate the production of this protective mucus through the lipopolysaccharides and short chain fatty acids they release.
In addition to their gatekeeper role, healthy intestinal bacteria also engage in a kind of chemical warfare with potential invaders. The microbiota are able to lower the pH of the intestinal cavity, which favours the growth of the good bugs over the bad ones. In addition, they produce hydrogen peroxide, a natural antimicrobial, and bacteriocins, a type of interspecies antibiotic.
As if that weren’t enough, the bacteria living in your colon actually ‘talk’ to your immune system. The dialogue helps educate the immune system so that it might operate more intelligently. How do bacteria do this? Their very presence ensures that the immune system has to stay alert. However, the short chain fatty acids butyrate and propionate, produced by gut bacteria, have also been shown to augment immune responses.
By fighting off bugs that would trigger an inflammatory response, and by directly augmenting the inflammatory response through communication with the immune system, gut bacteria can help to lower chronic inflammation. Various strains of gut bacteria have been shown to lower inflammatory markers, increase the presence of helpful immune cells and even help repair the gut barrier.
The study of gut bacteria is adding a new layer of complexity to our understanding of the immune system. Whether foods that feed bacteria (prebiotic), the bacteria themselves or their metabolic products (postbiotics), science is reexamining the role of bacteria in keeping us well – instead focusing only on those that make us sick.
Marchesi, Julian R., et al. “The gut microbiota and host health: a new clinical frontier.” Gut (2015): gutjnl-2015.
Giorgetti, GianMarco, et al. “Interactions between Innate Immunity, Microbiota, and Probiotics.” Journal of Immunology Research 2015 (2015).