Probiotic-based fermentation of watermelon waste: Effects on bioconversion efficiency, microbial shifts, and expression profiles of black soldier fly larvae.

Insects such as black soldier fly larvae (Hermetia illucens, BSFL) are efficient bioconverters whose growth and physiological performance are strongly influenced by diet composition, gut microbiota, and the molecular regulation. This study investigated how a probiotic-based fermentation strategy modulates larval physiology, microbiome dynamics, and gene expression when BSFL are reared on fermented watermelon waste. Watermelon waste was fermented for 14 d using a consortium of Bacillus subtilis, Enterococcus faecalis, and Aspergillus oryzae, resulting in a nutritionally enhanced substrate. BSFL fed on fermented diet exhibited significantly increased growth performance, biomass yield, and nutritional content of the insect biomass. Metagenomic analysis revealed marked enrichment of gut microbes belonging to genera known to include beneficial and commensal species (Enterococcus, Vagococcus, Carnobacterium, Tetragenococcus, and Blautia) along with a reduction in genera containing species previously associated with opportunistic or pathogenic traits (Mycobacterium, Pseudomonas, Morganella, Pedobacter, and Serpula), indicating diet-induced modulation of host-microbe interactions. Transcriptomic profiling highlighted an upregulation of key genes involved in growth and development (CK1, HIB, and PDK1), protein and fat biosynthesis (DVL, GSK3, and Lpin), and immune defense (PGRP-SA, Spz, Toll, and Cactus). Functional enrichment analysis further confirmed their participation in critical signaling pathways, including Hedgehog, Wnt, mTOR, Toll and Imd, and MAPK. Overall, this study demonstrates that probiotic fermentation improves nutrient utilization, regulates host-microbe interactions, and activates molecular pathways associated with growth and immune resilience in BSFL, providing new insights into the physiological and molecular basis of dietary adaptation in insects.