In May of 2000, heavy rains pummeled Walkerton, Ontario, a town of around five thousand people located a hundred miles northeast of Toronto. Excrement from cattle containing the bacteria E. coli and campylobacter washed into the municipal water supply. An epidemic of bacterial dysentery ensued; nearly half the residents became ill and seven people died.
Two years later, Canadian researchers began surveying the exposed population. One question they asked was: Since the outbreak, have you been told for the first time by a doctor or a health-care professional that you’ve developed depression, anxiety disorder, panic disorder, or post-traumatic-stress disorder? Residents who had diarrhea after being exposed to the contaminated water were more likely to answer yes than those who hadn’t had any gastrointestinal symptoms.
This association could simply represent a natural response to a serious illness. It’s often the case that if you get sick one year, you get depressed the next. Or, maybe, people who suffered gastrointestinal distress were inclined to respond affirmatively about any symptom—particularly given their pending lawsuits against the city. The researchers dealt with this potential bias by inserting a control question on ear-buzzing, for which the rates of the two groups were the same. This allowed the researchers to raise a third possibility: Could the bacteria itself cause depression?
A lot of public and scientific attention has been paid recently to the idea that the microbiome—the collection of bacteria, viruses, fungi, and other microbes that share our bodies, outnumbering our own cells ten to one—can cause diseases widely conceptualized as non-communicable. According to well-designed, peer-reviewed studies on rodents and humans, the microbiome appears to be a major contributor to obesity, diabetes, atherosclerosis, malnutrition, hypertension, asthma, rheumatoid arthritis, colon cancer, ulcers, inflammatory bowel disease, lymphoma, liver cancer, psoriasis, and even ear wax. We are, in many ways, a result of the organisms that live inside us. (Michael Specter wrote a feature on the microbiome in the magazine last year.)
But the link between the microbiome and how we feel and behave seems far more tenuous, if only because diseases of the mind are influenced by so many factors, and often elude clear biological pathways. A number of elegant studies, however, suggest that the microbiome may have as many implications for our brains and behavior as it does for more easily defined diseases.
At a recent National Institutes of Health conference on the topic, Ted Dinan, an avuncular, scholarly psychiatrist from Cork, Ireland, explained one way that bacteria in our gut could alter our behavior. Many organisms are capable of making neurotransmitters such as norepinephrine; nerves need to communicate with each other, and neurotransmitters serve as the key facilitators of this communication. Many people are familiar with the neurotransmitter serotonin, for instance, because it is targeted by widely used antidepressants, like Prozac. What many don’t realize, however, is that gut bacteria are actually the body’s major producer of serotonin.