Mind-Gut Connection: Why Intestinal Bacteria May Have Important Effects on Your Brain

Most children seem determined to eat dirt. It may be a coincidence, but what those little mud pie makers appear to intuit is now being supported by a growing body of scientific evidence that early exposure to diverse microorganisms results in healthier immune systems. Now there's reason to think that intestinal bacteria have important effects on brain development as well.

A study published in the March issue of Neurogastroenterology & Motility examined germ-free mice -- i.e. mice deprived of contact with bacteria at a formative age. The researchers observed changes in brain activity based on varying microbe levels. They also found germ-free mice more likely to engage in risky behavior -- measured as time spent in areas where they could be seen -- than mice with normal levels of intestinal flora.

The study concludes that this constitutes evidence of bacteria in the loop between belly and brain, and influencing behavioral development.

A separate study, published last November in Archives of General Psychiatry, surveyed the scientific literature for evidence of a connection between gut microbes and depression, and suggested that certain bacteria might be considered as treatment for depression.

Discoveries such as these support the decades-old "Hygiene Hypothesis," which postulates that hyper-sterile environments, widespread use of antimicrobial soaps, and general paranoia about bacteria are responsible for many so-called "diseases of civilization," like asthma, allergies, and other autoimmune disorders. The recently discovered importance of bacteria in brain function helps deepen understanding of our relationship with these ancient organisms.

Anybody who's ever "listened to their gut" when making important decisions might be satisfied to learn of this biochemical evidence for the mind/belly connection. The mechanism by which mouse-belly microbes might influence mice brains isn't known. There is speculation that the vagus nerve is a likely conduit.

The vagus nerve connects the brain to several parts of the digestive system. It's what tells your brain how hungry you are, based on what it senses in your belly. The vagus nerve has also been shown to carry signals initiated by bacteria. Staphylococcus can attack the vagus nerve and induce vomiting. Salmonella infections have been shown to affect brain activity, a connection lost when the vagus nerve is severed.

However mouse gut bacteria exert their influence on the brain, the fact that they do so, on top of all of the other cooperative relationships we've been discovering with bacteria, is amazing. And it makes me wonder who's really in charge. Are we simply hosting these creatures, or driving them around as well? When kids eat dirt, is it because the bacteria are telling them to?

Although we have a clear size advantage on the bacteria we harbor, they dramatically outnumbered us. And on the genetic level, bacteria bring far more to the table. Of the 3 million different genes identifiable in our bodies, only 30,000 are human genes. We share those additional millions of bacterial genes with thousands of different species.

The species lines in bacteria can be fuzzier than with mammals. I'm a lot less likely to exchange DNA with my dog than are two bacterium with each other. The genetic mixing is greatly assisted by bacteriophages: viruses that infect bacteria. Some of the most numerous and widely distributed creatures in the biosphere, bacteriophages are viruses that attack bacteria. They usually insert some of their own genetic information into the host, while helping themselves to what looks good in the host's genetic fridge. Then they move on to the next bacteria and do it again, spreading genes as they go. When we speak of bacterial populations it's a given that there is an associated bacteriophage population greasing the wheels of whatever's going on.