-Tryptophan suppresses enteric pathogen and pathobionts and prevents colitis by modulating microbial tryptophan metabolism.

-Amino acids (-AAs) have various functions in mammals and microbes. -AAs are produced by gut microbiota and can act as potent bactericidal molecules. Thus, -AAs regulate the ecological niche of the intestine; however, the actual impacts of -AAs in the gut remain unknown. In this study, we show that -Tryptophan (-Trp) inhibits the growth of enteric pathogen and colitogenic pathobionts. The growth of is strongly inhibited by -Trp treatment. Moreover, -Trp protects mice from lethal . infection via reduction of the pathogen. Additionally, -Trp prevents the development of experimental colitis by the depletion of specific microbes in the intestine. -Trp increases the intracellular level of indole acrylic acid (IA), a key molecule that determines the susceptibility of enteric microbes to -Trp. Treatment with IA improves the survival of mice infected with . . Hence, -Trp could act as a gut environmental modulator that regulates intestinal homeostasis.