Unraveling the Bioactivities and Immunomodulatory Potential of Postbiotics Derived from Bacillus subtilis and B. amyloliquefaciens for Aquaculture.

Postbiotics are molecules or soluble factors released as a result of a probiotic's metabolic activity. Their use in enhancing the growth, health, and disease resistance of aquatic animals has gained considerable attention in aquaculture. The present investigation was designed to assess the beneficial effects of postbiotic products derived from two probiotic strains, Bacillus subtilis and B. amyloliquefaciens. Postbiotics from B. subtilis exhibited significantly greater (p < 0.05) antibacterial activity against various pathogenic bacterial strains, more robust antagonistic growth kinetics, stronger anti-virulence potential, enhanced inhibition of biofilm formation, and increased antioxidant activity compared to those from B. amyloliquefaciens. Additionally, B. subtilis postbiotics triggered a significant (p < 0.05) cellular immune response, including higher myeloperoxidase activity, leucocyte proliferation, and production of nitric oxide and superoxide anions, along with a notable upregulation of immune-related gene expression (IL-1β, IL-10, IFN-γ, and TNF-α) in the head kidney leucocytes of Labeo rohita. A challenge test on L. rohita fingerlings confirmed the safety of B. subtilis postbiotics. These findings highlight the anti-pathogenic, immunostimulatory, and disease-resistant properties of B. subtilis postbiotics, suggesting their promising application in aquaculture.