BACTERIA-HUMAN BEHAVIOR INTERFACE. “We are just beginning to understand the wide influence exerted by the gut microbiota on brain and behaviour. However, it is already apparent that there is substantial overlap between the physiological relevance and behavioural impact of the gut microbiota, tryptophan metabolism and the serotonergic system.”
http://www.sciencedirect.com/science/article/pii/S0166432814004768