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