Derivatives of postbiotics (cell wall constituents) from Bacillus subtilis (LCBS1) relieve soybean meal-induced enteritis in bullfrog (Aquarana catesbeianus)

Soybean meal (SM) serves as a primary alternative to fish meal in aquafeeds. However, a high-SM diet may result in intestinal injury. Our previous study demonstrated the probiotic effects of heat-inactivated Bacillus subtilis (LCBS1) on bullfrogs (Aquarana catesbeianus) fed a high-SM diet, probably attributed to the bioactive constituent of cell wall. Therefore, in this study, the main constituents of cell wall from LCBS1, including peptidoglycan (PGN), lipoteichoic acid (LTA), cell wall protein (CWP), and whole cell wall (WCW), were extracted and added to a high-SM (~55 %) diet to investigate their probiotic effects on bullfrogs and reveal the possible mechanisms. The results indicated that bullfrogs fed the LTA of LCBS1 showed the highest weight gain, feed efficiency, and protein efficiency ratio. Additionally, the LTA of LCBS1 could activate the humoral immunity and modulate intestinal microbiota. It might activate JAK2-STAT3 and MAPK-ERK pathways, as well as up-regulate tlr5 gene to promote intestinal cell proliferation, thereby alleviating jejunal injury. The WCW of LCBS1 effectively increased the growth performance of bullfrogs by improving the humoral immunity, enhancing intestinal barrier function, and alleviating intestinal inflammatory response. The PGN and CWP of LCBS1 could stimulate the humoral immunity and enhance intestinal barrier function, but had no significant effect on the growth performance of bullfrogs. In conclusion, the LTA might be the primary bioactive constituent of heat-inactivated LCBS1, with the beneficial effects of promoting intestinal cell proliferation and enhancing intestinal barrier function, therefore alleviating the intestinal injury induced by SM on bullfrogs. This study establishes a theoretical basis for the efficient utilization of plant proteins by the application of postbiotics additive in aquafeed, which further saves the feed costs and promotes development of economically sustainable aquaculture.