It has been established that certain bacteria (such as Clostridial species) possess TRP decarboxylase, which converts TRP into the neurotransmitter tryptamine.167 The microbiome-derived tryptamine serves as a ligand for the gut epithelium-expressed serotonin receptor 4 (also known as 5-HT receptor 4, 5-HT4 receptor), contributing to heightened intracellular cAMP levels and increased fluid secretion into the gut to accelerate gastrointestinal motility,168,169 indicating therapeutic potential in gastrointestinal motility disorders such as irritable bowel syndrome (IBS). This evidence concerns the gene HTR4 and irritable bowel syndrome.