Host-derived Bacillus velezensis enhances intestinal structure, antioxidant capacity, and pathogen resistance in the American bullfrog (Aquarana catesbeiana).

The development of host-derived probiotics presents a forward-looking nutritional component strategy for sustainable aquaculture. However, research on host-associated probiotics for the globally farmed American bullfrog (Aquarana catesbeiana) is limited. The research put Bacillus velezensis (BLS) isolated from bullfrog guts determine its probiotic potential. Bullfrogs were divided into two groups: one fed a control diet and the other fed a diet enriched with 1 × 10⁸ CFU/g BLS for 7 weeks. BLS supplementation significantly enhanced growth performance and markedly decreased mortality after challenge with Streptococcus agalactiae. Relative to controls, the BLS cohort demonstrated heightened activity of essential antioxidant enzymes (CAT, GSH-Px, and SOD) and lower malondialdehyde levels in liver and intestinal tissues. Moreover, BLS supplementation enhanced intestinal structure, demonstrated by markedly increased villus height and goblet cell count, alongside enhanced hepatocyte arrangement and tissue integrity. Gut microbiome analysis revealed that BLS elevated the proportion of Firmicutes while decreasing the proportion of harmful genera such as Elizabethkingia. These findings suggest that the probiotic effects are mediated through the modulation of gut barrier function and microbial community. In conclusion, dietary supplementation with B. velezensis BLS enhanced growth, pathogen resistance, antioxidant capacity, and bowel health in bullfrogs through enhanced intestinal morphology and microbiome regulation. Consequently, supplementation holds significant promise as a probiotic in bullfrog farming.