A gut commensal bacterium promotes black soldier fly larval growth and development partly via modulation of intestinal protein metabolism

Black soldier fly (BSF), Hermetia illucens (L.) (Diptera: Stratiomyidae), is a promising bio-agent to transform organic wastes into valuable insect biomass, and the intestinal microbes play an essential role in this process. Here, we report that Citrobacter amalonaticus, a Gram-negative gut commensal bacterium of BSF larvae, can colonize the whole intestines of the larva and significantly promote the larval growth and development. The bacterial association with the larva had a marked impact on the larval transcriptome and modulated the expression of genes involved in dozens of biological pathways including host macro-nutrient metabolism and immune response. We investigated the underlying mechanisms driving the interaction and found that C. amalonaticus association can directly promote the expression of two gene families related to intestinal protein metabolism: Hitryp serine protease trypsin family and Himtp metallopeptidase family. To determine the gene function, we developed a symbiont-mediated double-strand RNA interference approach and successfully achieved gene knockdown in larval midgut in situ. Knockdown of the two gene families and protease inhibition in larval intestines attenuated the promoting effects on larval growth significantly. The results indicate that the gut symbiont promotes BSF larval growth partly via modulating the expression of the genes involved in intestinal protein metabolism. Taken together, this study establishes a novel genetic tool to study the insect functional genomics and the host-symbiont interaction and sheds light on the important roles of intestinal protein metabolism on the interaction. IMPORTANCE: Black solider fly larvae and the gut microbiota can recycle nutrients from various organic wastes into valuable insect biomass. We found that Citrobacter amalonaticus, a gut commensal bacterium of the insect, exerts beneficial effects on larval growth and development and that the expression of many metabolic larval genes was significantly impacted by the symbiont. To identify the larval genes involved in the host-symbiont interaction, we engineered the symbiont to produce double-strand RNA and enabled the strain to silence host genes in the larval gut environment where the interaction takes place. With this approach, we confirmed that two intestinal protease families are involved in the interaction and provided further evidence that intestinal protein metabolism plays a role in the interaction. This work expands the genetic toolkits available to study the insect functional genomics and host-symbiont interaction and provide the prospective for the future application of gut microbiota on the large-scale bioconversion.