Bacillus subtilis LCBS1 supplementation and replacement of fish meal with fermented soybean meal in bullfrog (Lithobates catesbeianus) diets: Effects on growth performance, feed digestibility and gut health

Two consecutive feeding trials were conducted to evaluate the effects of Bacillus subtilis LCBS1 supplementation and substituting fish meal (FM) with fermented soybean meal (FSM) in terms of growth performance, feed digestibility, intestinal morphology, and microbial composition of bullfrogs. In trial 1, bullfrogs received either a soybean meal (SM)-based basal diet (control check, CK group) or probiotic diet (basal diet was supplemented with 1 × 10 7 CFU/g of B. subtilis LCBS1, BS group). Each diet was fed to three replicates of bullfrogs (29.61 ± 0.07 g) for 58 days. Evaluation of B. subtilis LCBS1 FSM as a FM replacer in bullfrog diet was conducted in trial 2. A diet containing 20% FM formed the basal diet, and 60% and 100% of the FM was replaced with SM or FSM (FM, SM60, FSM60, SM100, and FSM100 diets). Each diet was fed to three replicates of bullfrogs (23.3 ± 0.3 g) for 63 days. As a result, in trial 1, BS treatments significantly increased weight gain (WG) and whole-body protein deposition. Significant improvements in villus height (VH), muscularis thickness (MT), and digestibilities of dry matter (DM), crude protein (CP) and phosphorus (P) were observed in the BS group compared with the control group. Probiotic supplementation resulted in elevated IL-4 and IL-10 expression and decreased TNF-α and IL-17 expression. A remarkable decrease in serum D-lactate concentration and diamine oxidase activity was detected in the BS group. Proteobacteria, Fusobacteria, and Firmicutes were the three most abundant bacterial phyla in each group, while Enterobacter and Bacillus were the most abundant genera in the CK and BS groups, respectively. In trial 2, compared with bullfrogs that received the SM diets, bullfrogs fed either the FM or FSM diets showed greater WG, feed efficiency, hind leg index, whole-body CP content, and digestibilities of DM, CP, calcium, P, and phytate P, and increased VH, villus thickness, and MT. Serum urea nitrogen and ammonia concentrations were significantly higher in the SM group than the FM or FSM group. Overall, dietary B. subtilis LCBS1 supplementation improved growth and digestion in bullfrogs and attenuated the intestinal structural damage and microflora dysbacteriosis caused by SM-based diets. Substituting 60–100% of FM with SM impaired the growth performance, digestion ability, and jejunal morphology of bullfrogs, while replacing equivalent FM with FSM effectively ameliorated these traits. • The phytase activity of a bullfrog intestinal autochthonous bacterial strain ( Bacillus subtilis LCBS1) was determined in vitro. • Dietary B. subtilis LCBS1 supplementation generally improved the growth and digestion of bullfrog. • The nutritional quality of soybean meal (SM) was improved by fermentation with B. subtilis LCBS1. • Replacing fish meal with fermented SM effectively ameliorated the SM-induced enteritis in bullfrog.