Since 2007, when scientists announced plans for a Human Microbiome Project to catalog the micro-organisms living in our body, the profound appreciation for the influence of such organisms has grown rapidly with each passing year. Bacteria in the gut produce vitamins and break down our food; their presence or absence has been linked to obesity, inflammatory bowel disease and the toxic side effects of prescription drugs. Biologists now believe that much of what makes us human depends on microbial activity. The two million unique bacterial genes found in each human microbiome can make the 23,000 genes in our cells seem paltry, almost negligible, by comparison. ‘‘It has enormous implications for the sense of self,’’ Tom Insel, the director of the National Institute of Mental Health, told me. ‘‘We are, at least from the standpoint of DNA, more microbial than human. That’s a phenomenal insight and one that we have to take seriously when we think about human development.’’The NIH has just awarded four grants of $1 million each on the relationship between gut microbes and mental health, so look for more announcements along these lines as research heats up.
Given the extent to which bacteria are now understood to influence human physiology, it is hardly surprising that scientists have turned their attention to how bacteria might affect the brain. Micro-organisms in our gut secrete a profound number of chemicals, and researchers like Lyte have found that among those chemicals are the same substances used by our neurons to communicate and regulate mood, like dopamine, serotonin and gamma-aminobutyric acid (GABA). These, in turn, appear to play a function in intestinal disorders, which coincide with high levels of major depression and anxiety. Last year, for example, a group in Norway examined feces from 55 people and found certain bacteria were more likely to be associated with depressive patients.
Will this lead to effective medicine? Actually we already know that a healthy gut is related to overall health, and that a healthy diet can give you a healthier gut; we also know that sometimes probiotics help some people. But beyond that we are talking about whole communities of thousands of species of microbes, so these are fiendishly complex interactions and just poking them with some drug is just as likely to do harm as to help.
Here's a question to ponder: what if it turns out that the community of microbes in our guts does influence our personalities, and that we can alter our personalities by messing with our bacteria? What then?