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. And 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, 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 a 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. At the least, the recently discovered significance of bacteria in brain function helps deepen our 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 a mind-belly connection.
The mechanism by which mouse-belly microbes might influence mice brains isn't known. There's 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. It's 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 are we 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 outnumber us. And on the genetic level, bacteria bring far more to the table. Of the three million genes identifiable in our bodies, only 30,000 are human. 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 some of the most numerous and widely distributed creatures in the biosphere: bacteriophages, 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, there's an associated bacteriophage population greasing the wheels of whatever's going on.
A recent study found large amounts of bacteriophage DNA in kimchi, suggesting a significant role for bacteriophages in the fermentation process. Kimchi is just one of many bacteria-rich (and presumably phage-rich) foods, like yogurt and sauerkraut, that many people consider to be superfoods. There are countless kinds of fermented foods in many diets, both old and new.
The Paleo diet—short for Paleolithic—is a modern diet that's based on the foods humans would have had access to during our evolutionary formative years. According to the Paleo diet, modern-day foods like sugar, grains, and processed carbohydrates shift the balance toward undesirable flora, while animal- and vegetable-based dishes, including fermented foods and vinegar, encourage good bacteria. It's no wonder, according to the Paleo worldview, that foods that have been with us since the beginning are the ones that keep our bodies in proper balance—with the help of our old friends bacteria.
The "Gut and Psychology Syndrome" (GAPS) diet (it's also the name of the related book) is built on the premise of a link between mental and intestinal health. The diet mixes probiotic supplements with a regimen of foods designed to tilt the playing field so the good bacteria take over.
Probiotic supplements are essentially "good bacteria" by the millions, in pill form. Doctors often recommend them after a round of antibiotics, which can kill the good bacteria in your body along with the bad.
Much of what we're learning in labs is validating ancient wisdom, like the importance of fermented foods. And along these bacterial lines, science is also finding an important function for an organ it once dismissed as a useless evolutionary relic: the appendix. Now they're realizing that the appendix probably has immunological functions related to the fact that it acts as a reservoir of spare bacteria in case your gut flora gets killed off or flushed out, say, in a nasty bout of diarrhea.
We're still at the beginning of the bacterial learning curve. A research team recently determined that humans can be classified into three categories depending on the type of bacteria in their guts. Such bacterial affiliation is found in all humans, and is unrelated to race or to the gut-bacteria type of one's parents. At this point we have no idea what this means, or where it will lead. But we can be sure that as we continue learning about our relationship with bacteria, kids will continue eating nature's probiotic wherever they can find it. And the more we learn, the smarter they seem.