A duo of scientists at the University of Oxford, UK, has proposed an evolutionary framework to understand why our gut microbiota affects the brain and behavior.
Oxford researchers Katerina Johnson and Kevin Foster argue that understanding why gut microbiome influences behavior requires a focus on microbial ecology and local effects within the host. Image credit: Open Clipart-Vectors.
Growing evidence indicates that gut bacteria — especially Lactobacillus and Bifidobacterium species — can influence our social behavior, anxiety, stress and depressive-like behavior.
“We know there are numerous possible mechanisms, including communication via the vagus nerve (major nerve linking the gut and brain), the immune system and hormonal changes, as well as the production of neuroactive chemicals by gut microbes,” said Dr. Katerina Johnson, from the Department of Experimental Psychology at the University of Oxford.
“But why should we expect gut bacteria to affect behavior at all?”
In their paper, published in the journal Nature Reviews Microbiology, Dr. Johnson and her colleague, Professor Kevin Foster from the University of Oxford’s Department of Zoology, consider the evolutionary pressures that may have led to ‘that gut feeling.’
One theory gaining momentum is that gut bacteria actively manipulate our behavior for their own benefit. For instance, they might change our behavior in ways that make us more sociable to increase their likelihood of transmission to new hosts.
Indeed, it is intriguing that numerous species of gut bacteria can produce chemicals of identical structure to our brain’s own neurotransmitters, or their precursors.
However, in light of evolutionary theory, Dr. Johnson and Professor Foster suggest this scenario, that our brains are manipulated by our microbes, is very unlikely given the immense diversity of microbial species and strains inhabiting the gut.
“Any extra energetic cost invested by bacteria producing a neuroactive chemical to manipulate host behavior would make it very vulnerable to being outcompeted by other microbes not making this additional investment,” Professor Foster said.
“The conditions favoring manipulation appear rarely satisfied by the genetically diverse ecosystem of the mammalian microbiome.”
“Rather than viewing our microbial companions as puppeteers manipulating our behavior, instead we suggest that the behavioral effects of gut microbes are more likely a result of natural selection on microbes to grow and compete in the gut, and natural selection on hosts to depend on their microbes,” Dr. Johnson said.
Microbial growth gives rise to metabolic by-products such as short-chain fatty acids known to affect brain function and microbial metabolites can also interact with our immune response.
“In addition, our physiology may have adapted to make use of our associated microbes,” the researchers said.
“Similar to the ‘hygiene hypothesis,’ which posits that an absence of microbes impairs immune system development, we propose that we may have evolved to depend on our microbes for normal brain function, such that a change in our gut microbiome could have effects on behavior.”
“An understanding of the evolution of gut-brain communication may help us to effectively engineer this microbial ecosystem with potential benefits for mental health and well-being.”
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Katerina V.-A. Johnson & Kevin R. Foster. Why does the microbiome affect behaviour? Nature Reviews Microbiology, published online April 24, 2018; doi: 10.1038/s41579-018-0014-3
