Serotonin (5-HT) signaling in the gut is influenced by the microbes and their byproducts. Bacteria that ferment dietary fibers produce short chain fatty acids (SCFAs) which can alter tryptophan hydroxylase 1 (TpH1) expression and activity as well as influence colonic motility through the release of 5-HT. Additionally, the full microbial community in the gut appears to influence the components of 5-HT signaling and motility, and can be induced or reversed by the presence or absence of gut bacteria.
Note: the bacteria in the diagram are copied from a Nature Reviews article, and are meant to serve as a guide to the medical illustrator who will ultimately create this figure.