Integrative effects of Bacillus tropicus FG2 on growth performance, immunity, gut microbiota, and metabolome in short-finned eel, Anguilla bicolor pacifica.

This study investigated the effects of dietary Bacillus tropicus FG2 supplementation at 0 (control), 107, 108, and 109 cfu g-1 diet (designated D7, D8, and D9, respectively) on growth, immunity, disease resistance, gut microbiota, and metabolomic profiles in eels. After 56 days, eels receiving the D9 diet exhibited significantly increased final body weight, weight gain, feed efficiency, and total biomass yield compared with the control and lower-dose groups, with no effect on survival. Intestinal protease, amylase and lipase, were markedly enhanced by probiotic supplementation. Immune responses were strengthened, evidenced by elevated serum lysozyme activity, upregulated expression of immune-related genes in the liver and spleen, and reduced mortality following Edwardsiella tarda challenge. Probiotic treatment also reshaped gut microbial communities, increasing beneficial taxa (Bacillus, Lactiplantibacillus, Cetobacterium somerae) while decreasing potentially pathogenic Clostridium. Metabolomic analyses revealed tissue-specific remodeling, with enrichment of amino acid metabolic pathways and antioxidant pathways. Overall, dietary B. tropicus FG2 enhances growth, digestive and immune functions, gut microbial balance, and metabolic homeostasis, supporting its application as a functional feed additive in eel aquaculture.