Psychosocial stress has long been known to affect the gut as well as its microbial residents. More than 10 years of research in rodents, monkeys, and human subjects have shown that even mild stress can transiently reduce the abundance of certain types of microbes, including the lactobacilli in the stool of the animals.
In chronic stress models, this decrease in lactobacilli has been shown to interfere with the metabolism of tryptophan, an important amino acid and a precursor of serotonin. Serotonin is not only essential for your gut health, but it also plays an important role in mood, pain sensitivity, sleep and other vital functions. Instead of metabolizing tryptophan to the beneficial serotonin, the stress-induced decrease in lactobacilli tryptophan is metabolized into a molecule called kynurenine, which plays a role in the inflammatory and degenerative changes in the nervous system. Even prenatal stress in pregnant mice has been shown not only to influence the mothers gut microbes, but also the gut microbiome of the offspring.
How does stress influence gut microbial abundance and function? For one, stress via its effect on the autonomic nervous system can change gut contractions, transit, and secretion of mucus and fluids. More surprising, stress can also have more direct effects on microbial behavior, via changing the expression of so-called virulence genes leading to some microbes being more hostile toward their host. This direct stress effect is mediated by the stress hormone norepinephrine, which is not only released into the bloodstream during a stressful situation, but also leaks into the gut and binds to the specialized receptors on gut microbes.
The findings by Patrick et. al. that the similar gut microbial changes occur in both “winners and losers” of a conflict is somewhat surprising. However, it is possible that there were differences in the metabolites that these stressed microbes produced, which are the language that microbes use in order to communicate with the host.
And keep in mind that these studies were performed in rodents; extrapolating the results to human behaviors should always be done so with caution.