First Attempt at Synthetic Microbial Communities Design for Rearing Gnotobiotic Black Soldier Fly Hermetia illucens (Linnaeus) Larvae

Previous studies have suggested correlations between the microbiota of the black soldier fly and larval growth and bioconversion ability, primarily through functional inference. However, the concrete impact of the microbiota remains to be demonstrated. To address this, we assembled two synthetic microbial communities (SynComs) derived from endogenous bacteria and evaluated their effects on larval growth. SynComs were administered to axenic larvae reared on sterilised diet (gnotobiotic) or as a probiotic in non-sterile treatments. Larvae were reared on vegetable-based (pre-consumer vegetable residues) or on animal-based (chicken hatchery residues) substrates. The SynComs were administered at two concentrations (5 × 10⁷ and 10⁸ CFU per isolate) in the substrate prior to neonate introduction. SynComs improved the growth of axenic larvae compared to untreated controls, although not to the levels observed in conventionally reared larvae. In non-sterile conditions, the combined SynComs increased growth on vegetable-based substrate, but no effect was observed on the animal-based substrate, suggesting a substrate-dependent effect. These results highlight microbiota's critical role in larval development and the potential of microbiome engineering in insect rearing systems. This preliminary study opens the way for optimisation of SynCom assemblies, which could be enhanced through pre-testing of individual isolates and selecting microbial combinations tailored to specific substrates.