Microbial tryptophan catabolism affects the vector competence of Anopheles

The influence of microbiota on mosquito physiology and vector competence is becoming increasingly clear but our understanding of interactions between microbiota and mosquitoes still remains incomplete. Here we show that gut microbiota of Anopheles stephensi, a competent malaria vector, participates mosquito tryptophan metabolism. Elimination of microbiota by antibiotics treatment leads to the accumulation of tryptophan (Trp) and its metabolites, kynurenine (Kyn), 3-hydroxykynurenine (3-HK) and xanthurenic acid (XA). Of these, 3-HK impairs the structure of peritrophic matrix (PM), thereby promoting Plasmodium berghei infection. Among the major gut microbiota in An. stephensi, Pseudomonas alcaligenes plays a role in catabolizing 3-HK as revealed by whole genome sequencing and LC-MS metabolic analysis. The genome of P. alcaligenes encodes kynureninase (KynU) that is responsible for the conversion of 3-HK to 3-Hydroxyanthranilic acid (3-HAA). Mutation of this gene abrogates the ability of P. alcaligenes to metabolize 3-HK, which in turn abolishes its role on PM protection. Colonization of An. stephensi with KynU mutated P. alcaligenes fails to protect mosquitoes against parasite infection as effectively as those with wild type bacterium. In summary, we identify an unexpected function of gut microbiota in controlling mosquito tryptophan metabolism with the major consequences on vector competence.