"But recently it has become clear that a third player is part of the game — the gut microbiota. Due to the vast neuronal network that is harboured by the gut wall and connected with the brain, intestinal bacteria and their metabolites not only can influence gut functions, such as motility and secretion, but, based on preclinical studies, may even influence emotional behaviours.
Another road through which gut bacteria may get in contact with brain circuits is the blood stream. Unsurprisingly, based on several recent publications on rodent studies, the hopes for improved future treatments of intestinal and possibly even psychiatric diseases are high. However, many of the details of this utterly complex interplay demonstrated in animal models remain to be investigated before research results can be translated into possible clinical applications."
Gut feelings: can the microbiota talk to the brain? - The Gut Microbiota For Health Experts Exchange