90% of the human body’s serotonin (also known as 5-hydroxytryptamine, or 5-HT) is produced by enterochromaffin (EC) cells in the gut and is used for enteric functions.

A paper in Cell by Yano and colleagues demonstrates that the microbiota play a vital role in regulating serotonin in the host. In the gut microbiota of mice and humans, spore-forming bacteria promote 5-HT biosynthesis from EC cells, and microbiota-dependent effects on gut serotonin modulate gastrointestinal motility and other functions.

The authors looked for the key metabolites in these processes, and identified certain ones — increased by the presence of spore-forming bacteria — that elevated 5-HT in EC cultures; they suggested the occurrence of direct metabolic signaling from gut microbes to EC cells. In addition, they found that in germ-free mice, elevated luminal concentrations of specific microbial metabolites increased 5-HT in the colon and blood.

Yano JM, et al. (2015) Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell. http://dx.doi.org/10.1016/j.cell.2015.02.047