Background
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 mechanisms remain to be thoroughly investigated.

Results
Here, we found that the microbiota was required forBactrocera dorsalislarval growth under poor diet conditions. Mono-association experiments revealed thatEnterobacteriaceaeand someLactobacillipromoted larval growth. Of 27 tested bacterial strains, 15 strains significantly promoted larval development, and theEnterobacteriaceae cloacaeN29 isolate exhibited the most obvious promoting effect. Bacterial genome-wide association study (GWAS) revealed that the vitamin B6 synthesis pathway was critical forE. cloacaegrowth promotion. The deletion ofpdxAresponsible for the vitamin B6 biosynthesis deprived the mutant strains of larval growth promotion function, indicating thatpdxAgene was crucial for promoting larval growth in the N29 strain. Importantly, supplementation of vitamin B6 to poor diet successfully rescued the axenic larval growth phenotype ofB. dorsalis.

Conclusion
Our results suggest that gut microbes promote insect larval growth by providing vitamin B6 under nutrient scarcity inB. dorsalis.