Dietary supplementation with Bacillus tropicus enhances growth performance, modulates gut microbiota, and improves health status in giant freshwater prawn, Macrobrachium rosenbergii

This study evaluated the impact of the probiotic Bacillus tropicus on growth, health parameters, and gut microbiota composition in Macrobrachium rosenbergii. Prawns were administered diets containing B. tropicus at concentrations of 6.45 × 10⁷ and 6.45 × 10⁸ colony-forming units (cfu)/kg over a 56-day feeding trial. These two diets were designated D7 and D8, respectively. Results demonstrated a significant improvement in feed efficiency and total biomass yield for prawns fed the D7 diet, although no significant differences were detected in final body weight or weight gain among the groups. Protease activity in the hepatopancreas was significantly elevated in both probiotic-treated groups, suggesting enhanced protein digestion capacity. Following challenge with Lactococcus garvieae, prawns fed probiotic-supplemented diets exhibited significantly higher survival rates, indicating improved disease resistance. Immunological assessments revealed that B. tropicus supplementation significantly enhanced lysozyme activity, phagocytic activity, and the expression of key immune-related genes (propo, lgbp, mnsod, and crus) in both hepatopancreases and hemocytes. Gut microbiota profiling revealed distinct microbial community structures across control and probiotic-treated groups, with a shift in dominant phyla and genera. Notably, the abundance of opportunistic pathogens such as L. garvieae and Enterobacter roggenkampii was markedly reduced in the gut of prawns fed the D7 diet, although alpha diversity indices remained unchanged. These findings suggest that B. tropicus supplementation confers multiple health benefits to M. rosenbergii, including improved growth performance, immunomodulation, enhanced resistance to bacterial infection, and modulation of gut microbiota composition. Thus, B. tropicus represents a promising probiotic candidate for sustainable prawn aquaculture.