Intestinal commensal bacteria promote Bactrocera dorsalis larval development through the vitamin B6 synthesis pathway

The gut microbiota can facilitate host growth under nutrient-constrained conditions. However, whether this effect is limited to certain bacterial species remains largely unclear, and the relevant underlying mechanisms remain to be thoroughly investigated. We found that the microbiota was required for Bactrocera dorsalis larval growth under poor dietary conditions. Monoassociation experiments revealed that Enterobacteriaceae and some Lactobacilli promoted larval growth. Among the 27 bacterial strains tested, 14 significantly promoted larval development, and the Enterobacteriaceae cloacae isolate exhibited the most obvious promoting effect. A bacterial genome-wide association study (GWAS) revealed that the vitamin B6 synthesis pathway was critical for the promotion of E. cloacae growth. Deletion of pdxA, which is responsible for vitamin B6 biosynthesis, deprived the mutant strains of larval growth-promoting function, indicating that the 4-hydroxythreonine-4-phosphate dehydrogenase(pdxA) gene was crucial for promoting larval growth in E. cloacae. Importantly, supplementation of a poor diet with vitamin B6 successfully rescued the axenic larval growth phenotype of B. dorsalis. Our results suggest that gut microbes promote insect larval growth by providing vitamin B6 under nutrient scarcity conditions in B. dorsalis.