Since the substantial, recent increase in attention towards the human gut microbiome, ample amounts of research has been put forth in pursuance of fully understanding the complex relationship between our metabolic processes and the commensal bacteria dwelling in the gastrointestinal tract. In realizing the many regulatory roles our microflora play in our most significant bodily mechanisms. As more information has accrued, researchers have turned their attention to the possibility that our hidden metabolic organ could influence our most protected organ: the brain.
Through the newly recognized microbiota-gut-brain axis, our microbes have established mechanisms by which they can exert their influence over the brain from their long distance domain. Having shown the ability to secrete numerous chemicals, researchers have further identified that a portion of those produced can be used to communicate with the brain via blood or neurons. In regards to the latter, the tenth cranial nerve, known as the vagus nerve, is the only cranial nerve to leave the brain and penetrate the diaphragm. This gives our microbes a direct line to our nervous system, facilitating action potentials via neurotransmitter release that can show direct consequence to emotions controlled by the central nervous system (1, 2).
A few of the chemicals released by our microbial communities that can regulate our mood (and the disorders caused by its alterations) are serotonin, dopamine, and gamma-aminobutyric acid (a.k.a. GABA). Differing the concentrations of these chemicals can control appetite-related signals, changing one’s perception of fullness and digestion. Manipulation of these chemicals can also contribute to the development of a variety of intestinal disorders that show strong correlation to depression and anxiety, as well as other complex mental disturbances, often treated by medications that oppose the effects relayed by the bacteria (2).
Communication between the gut and the brain is not limited to one direction, but is in fact a bidirectional relationship. Mental status generated by the structures and chemicals within the brain can also have an effects on the strains of bacteria that constitute our gut microbiome. In particular, studies have been dedicated to showing how stress can impact microbe populations. Resulting alterations to those present have yielded lowered immune systems (a system that microbes contribute to), decreased protection from gastrointestinal diseases, etc.
In both directions, the relationship can be beneficial or harmful. Detrimental diseases of the psyche can lend to intestinal consequences or be caused by the bacteria there. On the same note, a strong microbiome can help improve mental status as well.
(1) Mayer EA, et al. 2014. Gut Microbes and the Brain: Paradigm Shift in Neuroscience. The Journal of Neuroscience, 34(46): 15490-15496
(2) Smith PA. 2015. Can the Bacteria in Your Gut Explain Your Mood? New York Times Magazine, https://www.nytimes.com/2015/06/28/magazine/can-the-bacteria-in-your-gut-explain-your-mood.html
