More and more studies show that the bacteria that inhabit our intestines are capable of producing effects on our behavior and mood.
The intestinal microbiota, especially some species of the Lactobacillus and Bifidobacterium genera, could influence our social behavior, anxiety, the response to stress and depression. How is it possible? ” Many mechanisms are known, one of them is through the vagus nerve, which communicates the intestine and the brain, and also through hormonal changes and in the immune system ,” explains Katerina Johnson, researcher at the Department of Experimental Physiology at the University of Oxford. She and her partner Kevin Foster, from the Department of Zoology at the same university, have just published an interesting article in the journal Nature Reviews Microbiology in which they discuss the possible evolutionary meaning of these surprising effects.
One of the most widely accepted theories today posits that microbes ‘manipulate’ our minds for their own benefit. For example, bacteria could change our behavior to make us more social and thus more likely to spread to other hosts. In fact, numerous intestinal microorganisms produce chemicals with the same structure as some of the neurotransmitters – or their precursors – that are formed in our brain.
However, let’s think a little more slowly about this hypothesis: if we were in a scenario in which there was only one species of bacteria living in our digestive system, the theory would make all the sense in the world, since the energy effort involved in producing these molecules would have their payoff, and the bacteria in question would be the only beneficiary. But we know that this is not the case: our intestines are home to an enormous diversity of microbial species, and taking this into account, the species that produce these compounds would be benefiting the entire community. In short: they would be making it very easy for their competitors, who would take advantage of the positive effects of altering human behavior without having made the slightest effort.
“Any additional energy costs invested by bacteria that produce neuroactive chemicals to manipulate host behavior makes them very vulnerable to competing with other microbes that have not made that additional investment,” explains Foster.
We depend on our bacteria
What the researchers propose is to turn the tables and approach the matter from the point of view of the selective pressures that affect, not bacteria, but their hosts, in this case humans. According to his theory, the production of these neuroactive compounds is not an end in itself, but rather they are secondary products formed as a result of the metabolic activity of some species of bacteria, and natural selection would have acted on us in the way that we are dependent on these metabolites.
“Rather than viewing our microbial companions as puppeteers who manipulate our behavior, we suggest that their effects on behavior are rather the collateral result of the activity they develop to grow and compete in our gut , and that, nevertheless, selection natural in the hosts will depend on their microbes ”, details Johnson. “Microbial growth gives rise to metabolic by-products such as short-chain fatty acids, which are known to alter brain function, and other microbial metabolites can also alter our immune response.”
“Our physiology seems to have adapted to make use of our associated microbes. In the same way that the hygiene hypothesis postulates that the absence of microbiota hinders the development of the immune system, we propose that evolutionary pressures have operated in our species in such a way that we depend on intestinal bacteria to develop normal brain function , and that a change in our gut microbiome could have effects on behavior ”.
Researchers highlight the importance of understanding well this evolution of gut-brain communication , since it is information that can help us manage this microbial ecosystem to obtain benefits on our health and mental well-being.
Reference: Johnson and Foster, 2018. Why does the microbiome affect behavior? Nature Reviews Microbiology doi: 10.1038 / s41579-018-0014-3
