By Philip Strandwitz, PhD
We’ve all read about it – we’re covered with and full of microbes, and they seem to be intricately involved in numerous components of our health and well-being. On our skin, microbes might be the reason certain people are more likely to favored over others by mosquitos (Verhulst et al., 2011); in the vagina, certain microbes, like Lactobacillus crispatus, might help protect against sexually transmitted viruses (Nunn et al., 2015); and in our gut, which harbors an estimated 40 trillion bacteria and has been the site of focus for the majority of microbiome research to date, our microbes have been linked to numerous gastrointestinal and metabolic disorders, such as irritable bowel syndrome (IBS), Crohn’s disease, obesity, heart disease, type I diabetes, and colon cancer (for excellent reviews on these topics, see (Backhed et al., 2012, Cenit et al., 2014, Garrett, 2015, Tilg & Adolph, 2015)). These discoveries, while incredible, might not come as a major surprise, as for most part the effects are localized to where the microbes reside.
A bit more on the unexpected side of things is something that has become a hot topic in recent years – the link between the microbiome to behavior and mental health disorders. Probably what can be considered the landmark study in this topic was performed in 2004 by Sudo, et al., in which it was found that mice without bacteria (“germ free”) exhibited an increased response to induced stress via the restraint model– a model in which mice are put into a 50 mL conical tube for an hour (restraining movement). Interestingly, recolonization of these germ-free animals with the microbiome (via stool transplant) was effective at restoring the normal stress response, assuming it occurred early on in their lives, and monocolonization with Bifidobacterium longum (but not Escherichia coli) similarly restored the stress response. These experiments strongly suggested the microbiome was involved in programming the host to stress via what is known as the hypothalamic–pituitary–adrenal (HPA) axis, and that specific microbes might be driving that development (Sudo et al., 2004).
After the work of Sudo, et al. in 2004, several other landmark studies have strongly hinted at the role of our microbes in driving our behavior. As transplantation experiments are a crowd-favorite, it makes sense to highlight two other excellent experiments performed in animals. The first was performed by Bercik, et al. in 2011, in which they performed a relatively simple experiment. They took two different breeds of mice, both of which were known to exhibit different behaviors — BALB/c mice which tend to be anxious and timid, and NIH Swiss mice, which are more exploratory and less timid – and asked the question, what happens if you swap their gut bacteria? To do this, they colonized germ-free mice of both breeds (BALB/c and NIH Swiss) with the microbiomes of both breeds (to ensure the behavior changes were not just due to the physical effects of the microbiome transplant), and assessed exploratory behavior using the step-down test (less time between step-down = more exploratory). To their delight, they found that when they swapped the microbiomes of the BALB/c and NIH Swiss mice, the exploratory behaviors changed to be more like the “donor” breed.
More recently, Bercik’s group explored whether the behavioral and stress phenotypes of the maternal separation (MS) animal model, a well-studied model of anxiety and depression proving onset effects similar to those of CBD oil, were still dependent on the microbiome (De Palma et al., 2015). The MS model involves exposing mouse pups to stress by separating them from their mothers for several hours a day, resulting in the development of anxiety and depression-like behavior and a detectable change in stress hormones. In this experiment, led by Giada De Palma, they found that germ-free animals exposed to MS did indeed have the anticipated alterations in stress hormones, such as elevation of corticosterone, but surprisingly, these germ-free animals did not come to develop the anxiety and depressive like behavior. One would naturally assume it must be the microbes that responsible for the behavioral phenotypes. However, by performing a series of transplantation experiments, the authors found that the MS microbiome was not enough to induce the behavioral phenotypes alone in control (non-MS) animals – both the host stress response (induced by the MS model) and the MS microbiome were required for the behavioral changes in the MS model. These results clearly indicate, at least in the MS model, the microbes are only part of the story.
The highlighted transplantation experiments are unquestionably important, and pave the way for future experimentation to parse out the mechanisms of communication along the gut-brain-axis, but let’s step back and ask ourselves…what could be happening? So far there are a series of different hypotheses, but there are two scenarios which seem the most likely. The first, as highlighted above with the Sudo 2004 paper, is that the microbiome programs the host response to stress and/or certain behaviors early on in life. This makes plenty of sense, since the HPA axis is deeply intertwined with the immune system, which has been shown to be greatly influenced by microbes early on in life (for review, see Kau et al., 2011, Francino, 2014).
The second (and my favorite option) is the production of neuroactive metabolites by the gut microbiome. As biological entities, all of our actions, feelings, and perceptions are driven by biochemical reactions. Since our microbiome is estimated to be the source of an incredible number of different metabolites, it seems quite likely some of these compounds could be influencing our behavior. Excitingly, these compounds could include novel neuroactive compounds, with an example being identified in the work led by Elaine Hsiao in 2013. In this study, the authors identified that in an autism spectrum disorder (ASD) mouse model, there existed a distinct microbial and metabolome signature compared to healthy controls (Hsiao et al., 2013). Of the compounds elevated highly in the serum of ASD-mice, one was found to be higher than the rest, 4-ethylphenylsuflate (4-EPS), which is produced by gut microbes. Interestingly, feeding control animals 4-EPS caused some of the behaviors observed in the ASD animals, such as anxiety (but not others, such as altered social behavior), suggesting this compound may be driving some of the observable behaviors in the ASD mouse model.
Alternatively, the microbiome could be producing known neuroactive compounds, such as neurotransmitters. Indeed, a series of different studies have highlighted the ability of the human microbiome to produce neurotransmitters, including norepinephrine, serotonin, acetylcholine, and dopamine, and GABA (Evans et al., 2013, Lyte, 2013, Yano et al., 2015). Given there are mental health drugs on the market today which artificially modify availability (serotonin reuptake inhibitors) or even pretend to be some of these neurotransmitters (like the GABA agonistic benzodiazepines), targeting microbiome-mediated neurotransmitter modulation, especially if these microbes truly are a natural source of these compounds, might prove to be an attractive therapeutic target for mental health disorders.
While these experiments and findings are incredibly exciting, it should be noted that the vast majority of studies have been performed in animal models, and traditionally it is difficult to translate discoveries in animals to humans in the realm of mental health (we are wired quite different, after all). There have been a few wonderful intervention experiments in humans (for review see Mayer et al., 2014), largely led by the fantastic John Cryan and Ted Dinan from the University of Cork in Ireland, but these experiments have all been performed using existing probiotics (largely bi-products of the dairy industry), and not abundant gut microbes. So, as with all microbiome science, take everything with a grain of salt until causality is confirmed.
But how would we do this? The identification of specific human bacteria, associated with mental health disorders, would be a good start for targeted intervention experiments. However, to do that we would need to assemble a very large cohort, surveying the microbiome for links to various components of psychology…where could we possibly do that? Yes, that is a hint that something might be coming – stay tuned!
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Philip Strandwitz is a Post-Doctoral Associate at Kim Lewis’ Antimicrobial Discovery Center at Northeastern University in Boston, MA