Gut microbiota-mediated recycling of residual nitrogen in black soldier fly larval frass

Black soldier fly larvae-based waste bioconversion systems generate frass containing substantial residual and excreted nitrogen, limiting overall nitrogen use efficiency. This study evaluates a microbiome-based strategy to enhance nitrogen recovery from black soldier fly larvae frass and redirect residual nitrogen into larval biomass. Using surface-sterilized larvae, targeted supplementation with gut-derived bacteria, and ¹⁵N-labelled urea tracing, nitrogen transfer from frass into larval biomass was quantified. Among eight isolated gut bacterial strains, Providencia rettgeri, Proteus mirabilis, and Citrobacter freundii showed the strongest individual effects on larval growth and nitrogen assimilation. A defined consortium of these strains consistently outperformed single-strain treatments, increasing larval dry mass and total nitrogen content by up to 49% and 58%, respectively. Isotopic analysis confirmed enhanced incorporation of urea-derived nitrogen into larval tissues, while residual nitrogen in frass was reduced by 29% relative to conventional rearing. Genomic analysis and urease inhibition experiments further demonstrated that bacterial urease activity underpins this enhanced nitrogen recycling. While demonstrated on a model substrate, the findings highlight potential adaptability to complex industrial wastes pending further validation. Targeted gut microbiome engineering via a urease-active consortium offers a biologically precise route to improve nitrogen use efficiency in insect bioconversion systems.