Probiotic Bacillus subtilis enhances silkworm (Bombyx mori) growth performance and silk production via modulating gut microbiota and amino acid metabolism.

BACKGROUND: Artificial diet-reared silkworms (Bombyx mori) exhibit reduced gut microbial diversity and impaired growth performance compared to mulberry-fed counterparts. While Bacillus subtilis is widely used as a probiotic in livestock and aquaculture, its impact on silkworms remains unexplored. This study investigates whether dietary supplementation with B. subtilis enhances larval development and elucidates the underlying mechanisms involving gut microbiota and metabolic pathways. RESULTS: Supplementing artificial diets with B. subtilis (6 × 105 CFU/g) significantly increased larval body weight by 9.1-22.1% during instar stages and improved feed utilization efficiency (FUE) by 4.09%-6.80% compared to controls. Cocoon quality metrics, including cocoon shell weight (+ 9.77% in females) and cocoon shell ratio (+ 6.56%), also improved. Mechanistically, B. subtilis did not colonize the midgut but transiently modulated gut physiology: it elevated midgut fluid pH and enhanced α-amylase, trypsin, and lipase activities. 16 S rRNA sequencing revealed reduced gut microbial diversity (Shannon index, P < 0.01) and shifts in community structure, with decreased abundances of potential pathogens (e.g., Pseudomonas) and commensals (e.g., Lactobacillus). Targeted metabolomics identified a 3.1-fold increase in phenylalanine levels in hemolymph, linked to upregulated aromatic amino acid metabolism pathways (KEGG). Dietary phenylalanine supplementation (0.4%) replicated B. subtilis-induced growth promotion, confirming its pivotal role in host-microbe interactions. CONCLUSIONS: B. subtilis enhances silkworm growth and silk production through multi-faceted mechanisms: reshaping gut microbiota composition, improving digestive enzyme activity, and elevating phenylalanine biosynthesis. These findings establish B. subtilis as a promising probiotic for optimizing artificial diet systems in Lepidoptera and highlight the central role of amino acid metabolism in insect-microbiome symbiosis.