In a world buzzing with political and economic uncertainty, anxiety and depression have run rampant in our society; the most recent studies estimate that 40 million Americans — or roughly 18% of the American population — are affected by the detrimental affects of anxiety (1). And with increasing research and curiosity surrounding the mysterious microbial inhabitants of our gut, ongoing research continues to unravel their not-so-subtle involvement in our well-being.
It is well established in the research community that our microbiome has profound affects on our weight, the production of vitamins and food digestion. But is it too far-fetched to believe that microscopic life-forms living in our intestines influence our brain functions? Can bacteria communicate with our brains to determine our moods, feelings and emotions?
It seems likely.
Physiologically speaking, there is no reason why this communication can’t happen. Of the billions of nerves that thread through our bodies, one of them — termed the “vagus nerve” — serves as a direct connection between the intestines and the brain. This connection, which is formally referred to as the “gut-brain axis”, has been a critical area of research for more than a decade. What we have discovered in that time is that the vagus nerve seems to be bi-directional, meaning that while the brain can act on on the gastrointestinal tract to shape its bacterial composition, at the same time gut microbes can produce certain chemicals that act on the brain (2).
Initial excitement was sparked in 2004, when a landmark study observed that mice that were devoid of bacteria (i.e. “sterile” or, more formally, “germ-free” mice) demonstrated heightened levels of stress hormones in their blood (compared to normal, bacteria-carrying mice) when placed in stressful conditions (3). What’s really interesting is that while the stress response was clearly dysfunctional in germ-free mice, researchers could induce a more normal hormonal response by simply treating the mice with one single microbe: a bacterium called Bifidobacterium infantis. This served as the first clue that our gut bacteria are doing something that influences our mood. Likely, gut bacteria produce chemicals that influence the brain to reduce psychological stress. These chemicals have since been coined as “psychobiotics”.
Further experimentation continued to shed light on the great influence our bacteria has on our mood. Prominent researchers Premsyl Bercik and Stephen Collins working out of McMaster University discovered that behaviour can be transferred between mice simply by transferring their gut bacteria. They discovered that if they took one species of bacteria from the intestines of a mouse, and gave that single strain to a germ-free mouse (such that the intestines of the recipient mouse only contained that single bacterial species), then the recipient mouse would also take on aspects of the donor mouse’s personality. For example, mice that were naturally timid/shy before receiving bacteria became more exploratory if they received bacteria from a naturally exploratory donor mouse. As another example, naturally daring mice would become more shy once they received the bacteria from a naturally shy mouse (4).
While this is only the beginning steps in what promises to be a long journey of intensive, complex research, these kinds of tendencies hint that microbial interactions could very well influence the brain to induce anxiety and other mood disorders.