Photo by Laura Rosa Brunet
It is estimated that bacterial cells out-number your cells ten to one. Sears (Anaerobe, 2005) claims that about 5,000 to 10,000 different species make themselves at home on the vast continent that is your body. So what effect do these "guests" have on you? Traditionally they have been credited with aiding digestion and competitively excluding pathogens, but a recent report suggests that at least one species of bacteria may improve your mood.
Following up on anecdotal reports that cancer patients treated with Mycobacterium vaccae claimed improvements to their moods, Dr. Chris Lowry and team treated mice with M. vaccae, and found that the bacterial treatment led to increases in systemic serotonin. Lowry et al. speculated that immune activation by the bacteria led to the increased activity of serotonergic neurons in the interfascicular part of the dorsal raphe nucleus. Since increasing serotonin levels is the mechanism of action for most antidepressants, the team suggested that M. vaccae may have effects in humans similar to that of antidepressants.
Given that behavioral changes concomitant with microbial infection are not unprecedented (viz. Toxoplasmosis gondii, see also The Loom), it does seem plausible that M. vaccae could alter human serotonin levels and reduce depression, but further experimentation is required to test this hypothesis. It even seems plausible that this enhances M. vaccae's fitness. Perhaps your emotional state is the consensus of imputs from various sources, environmental, nutritional, microbial etc. I think there is a whole new field of science waiting to emerge with regards to human community ecology.