Bacteria and Human Behavior: Peering Into the Universe Within You

Most of us are in the habit of viewing ourselves as human beings, or if not as humans at least as mammals. But science increasingly challenges this obvious self-perception, suggesting instead that while we may be humans, we are not fully mammalian. Rather, our apparent phylogenetic unity masks the deeper truth that we are conglomerates of many different life forms all pulling together to make us who and what we are. 

In fact, only one in every ten cells in your body is mammalian. By cell count each of us is about 90% bacterial.

This massive bacterial presence is found wherever our mammalian selves meet the outside world: the skin, the lungs, the mouth and nose, but especially the gut, where trillions of bacteria live together in what is widely recognized as the most complex ecosystem known in the universe.

Commonly referred to as the microbiota, this bacterial presence is increasingly recognized as having a mind of its own, if by a mind we mean the ability to impact who we are, how we feel, and how we behave. In fact, much of who we think we are may not be “us” at all.

A recent study highlights the profound impact the bacterial world may have on our mental and physical health.

In this study, researchers identified pairs of identical twins in which one of the twins was obese and the other was thin. They transferred gut bacteria from these individuals into specially-bred mice that have no gut bacteria of their own. These “gnotobiotic” mice have the ability to take up human microbiota and make it their own. Remarkably, mice colonized by gut bacteria from the obese twins promptly began to gain weight and become fat, whereas mice colonized with gut bacteria from the thin twins remained lean.

As remarkable as these findings are, and as relevant to psychiatry given the strong association between obesity and depression, they are part of a little-known larger picture that is in some ways even more striking.

For example, just as the composition of your microbiota may powerfully influence whether you are fat or thin, gut bacteria may also contribute to whether you are calm and happy, or anxious and sad.

Although it has yet to be shown in humans, several animal studies have demonstrated that it is possible to completely change the behavior of rodents not by transplanting their brains but by transplanting their gut contents. Indeed, if fecal matter (and hence microbiota) from an anxious mouse strain is transplanted into young mice from a calm strain, the little rodent recipients lose the behavioral legacy of their mellow genes and become highly anxious.

The opposite occurs when gut bacteria from calm strains are transplanted into juvenile mice from highly anxious strains: the formerly anxious mice become calm and confident. Thus, in a very real way who the mice are depends not on the mice themselves as we commonly think of them (or their genes), but rather, and to a large degree, on the composition of their bacteria.

If these types of jaw-dropping discoveries are shown to be anywhere as relevant for humans as they are for rodents, they will force us to think long and hard about the kind of beings we really are.

On a philosophical level, it may be that we are better seen not as distinct entities in the world but as what happened when bacteria decided they wanted to become conscious and take a look around.

On a clinical level, we have to ask ourselves whether we can be far off from stool transplantation as a treatment for at least some psychiatric diseases.

All of this is not lost on the National Institute of Mental Health, which has issued a call for grant submissions designed to investigate just how the gut microbiota manage to pull off these amazing behavioral feats. In the meantime, many of us are beginning to wonder whether, in a very real way, we are what we poop.

References

1. Ridaura VK, Faith JJ, Rey FE, et al. Gut microbiota from twins discordant for obesity modulate metabolism in mice. Science. 2013 Sep 6;341(6150):1241214. doi: 10.1126/science.1241214.

2. Bercik P, Denou E, Collins J, et al. The intestinal microbiota affect central levels of brain-derived neurotropic factor and behavior in mice. Gastroenterology. 2011 Aug;141(2):599-609.

Charles L. Raison, MD, is an associate professor in the Department of Psychiatry, College of Medicine and the Barry and Janet Lang Associate Professor of Integrative Mental Health in the Norton School of Family and Consumer Sciences, College of Agriculture and Life Sciences, University of Arizona, Tucson, AZ. He is also the behavioral health expert for CNN.com, and he is a Psych Congress Steering Committee member.

The views expressed on this blog are solely those of the blog post author and do not necessarily reflect the views of Psych Congress Network or other Psych Congress Network authors.