Biological Psychiatry

Earlier studies have shown that microbiota, a community of microorganisms in the gut, can influence behavior and emotion. Rodent models have demonstrated the effects of gut microbiota on emotional and social behaviors, such as anxiety and depression, but there has been little scientific evidence in humans.
https://psychcentral.com/news/2017/07/05/gut-bacteria-may-be-linked-to-mood-behavior-in-healthy-humans/122846.html