Dietary Probiotic Bacillus subtilis AAHM-BS2360 and Its Postbiotic Metabolites Enhance Growth, Immunity, and Resistance to Edwardsiellosis in Pangasianodon hypophthalmus

Edwardsiellosis, caused by Edwardsiella tarda, poses a significant threat to the aquaculture industry, particularly in pangasius farming. This study investigates the effects of probiotic Bacillus subtilis AAHM-BS2360 and its postbiotic metabolites on growth performance, immune responses, antioxidative activity, and disease resistance against E. tarda infection. A total of 240 healthy pangasius (37.0 ± 4.9 g) were divided into four treatment groups with four replicate tanks each, as follows: (1) the Control group, which received feed top-dressed with 100 mL of 0.85% NaCl/kg diet; (2) the Probiotic group, which received feed supplemented with 100 mL of B. subtilis AAHM-BS2360 cells at the concentration of 1 × 1012 CFU/kg diet; (3) the Postbiotic group, which received feed supplemented with B. subtilis AAHM-BS2360 cell-free supernatant 100 mL/kg diet; and (4) the Pro + Post group, which received a combination of B. subtilis AAHM-BS2360 cells and cell-free supernatant. After 30 days of feeding treatment, biochemical serum analysis revealed a significant increase in the AST/ALT ratio in the Postbiotic group. The Probiotic and Postbiotic treatments increased lysozyme activity in mucus, indicating an innate immune response to pathogens. The Pro + Post group exhibited the highest levels of catalase (CAT) in serum and upregulated antioxidant-related genes. All treatment groups receiving B. subtilis AAHM-BS2360, metabolites, and their combinations showed significant upregulation of immune-related genes, like lygl1, tgfb, b2ml, and tnf. The expression of proinflammatory genes (litaf, ifngr1l, c3, il13, and il1b) increased, with the most pronounced effects observed in the Pro + Post group. The Probiotic group showed significant upregulation of the growth-related gene igf1. Meanwhile, the Pro + Post group showed significantly higher values in SGR and ADG parameters, with values of 3.29 ± 0.98%/day and 1.42 ± 0.52 g/day respectively (p < 0.05). Survival rates were significantly higher in the Pro + Post (87.5%), Postbiotic (84.37%), and Probiotic (81.25%) groups when challenged with E. tarda. Dietary supplementation with B. subtilis AAHM-BS2360, its metabolites, and their combination enhanced immune response, reduced oxidative stress, and improved growth performance in pangasius, highlighting its potential as a functional feed additive for sustainable aquaculture.