Another possibly signifcant link between gut microbiota and behaviours is serotonin, or 5-hydroxytryptamine (5-HT), which is involved in the modulation of a variety of physiological and psychological processes13,14. Gut microbiota can both produce and modulate the host’s biosynthesis of serotonin15,16. Using germ-free animal models, Yano et al. elegantly demonstrated that microbiota promoted 5-HT biosynthesis from the colonic enterochromafn cells (EC)15, although animal experiments suggest that fecal serotonin can have a pro-infammatory role and stabilise the gastro-intestinal barrier17.