Probiotic Bacillus strains enhance cellulose digestibility and silkworms growth via gut microbiota modulation

Dietary supplementation with probiotics has been demonstrated to enhance nutritional efficiency and economic traits in the silkworms (Bombyx mori), with mechanistic insights indicating that these benefits are associated with gut microbiota activity and mucosal immune modulation through the regulation of enzymatic functions. This study investigated the effects of probiotic Bacillus species on intestinal microbiome diversity in fifth-instar silkworms. Two native isolates, Bacillus subtilis Y11 and Bacillus velezensis Z55, were administered either individually or in combination to experimental groups (CK, Y11, Z55, and Y11/Z55) on the first day of the fifth instar. Intestinal contents were collected at days 1, 3, and 5 after treatment for 16S rRNA gene sequencing, and cellulose content in silkworm excrement was measured on a daily basis. Experimental groups exhibited significantly elevated body weight, digestive capacity, and cocoon weight compared to controls. The cellulose content in silkworm excrement decreased by 24.7% and 31.8% on days 2 and 3, respectively, compared to the control (CK) group at the corresponding time points. Tax4Fun2 analysis further predicted a rapid increase in the potential abundance of cellulose degradation-related genes within the gut microbiome, an inference based on taxonomic composition. These findings support the hypothesis that probiotic treatment may enhance the net efficiency of cellulose degradation in silkworms by modulating the gut microbial community and structure, thereby offering new insights into improving their growth performance.