Bacillus siamensis alters gut microbiota and the TCA cycle during bioconversion of swine manure by black soldier fly larvae.

Black soldier fly larvae (BSFL, Hermetia illucens) is recognized as a promising insect species for converting organic waste into valuable biomass. However, when using specific substrates such as pig manure for breeding, the final nutritional quality of the larvae, especially the composition of limiting amino acids and fatty acids, often fails to meet the standards of high quality feed. This study aimed to investigate whether Bacillus siamensis MZ16 was associated with changes in the biotransformation efficiency and nutritional quality of BSFL in relation to the intestinal microbiota. Results showed that the MZ16 group had significantly higher the BSFL body weight (+13.97%) and the bioconversion rate of the swine manure compared with the control group (p < 0.05). More importantly, the nutritional composition of the BSFL differed significantly between the MZ16 and control groups, with increased level of key limiting amino acids methionine (+209.51%) and lysine (+101.41%) (Met and Lys) and the functional fatty acid lauric acid (C12:0) (p < 0.05). Multi-omics analysis suggested that MZ16 was associated with changes of the larval gut microbiota characterized by enrichment Clostridium_sensu_stricto_1 and suppression potentially harmful genus Providencia, alongside alterations in fiber degradation and long-chain fatty acid production, while shifts in TCA cycle were correlated with increased levels of lysine and methionine. Integrated analysis pointed to a potential role of Clostridium_celatum_DSM_1785 in these metabolic changes.