Abstract

Postbiotics are functional metabolites derived from bacteria that have been shown to promote growth, alleviate stress in aquatic organisms, and enhance the host immune system. They offer unique advantages over traditional probiotics in terms of safety and gastrointestinal stability, highlighting their tremendous potential in aquaculture. However, the variety of postbiotics formulations available for aquaculture and the research on their effects on host growth and disease resistance remain limited. In our study, we utilized a potentially applicable aquaculture strain of Bacillus velezensis to produce postbiotics and evaluate their modulatory effects on growth and immune regulation in largemouth bass. Our results demonstrate that supplementation with an appropriate concentration of postbiotics significantly promotes growth and gut health in largemouth bass, with the most pronounced effects observed in the 50 mL/kg group. Moreover, postbiotics feeding induced the expression of innate antiviral genes ( dhx58 , trim25 , mx1 , and ifnγ ) and adaptive immune-related genes ( igm , igt , cd8α , and cd4 ) in the gut and head kidney and markedly enhanced the activities of serum innate immune enzymes, including lysozyme, superoxide dismutase, and acid phosphatase. Importantly, postbiotics supplementation increased the proportion of IgM + B cells and IgM secretion, thereby activating humoral immune responses and mediating enhanced antiviral defense in largemouth bass. In conclusion, this study systematically investigated the optimal effective concentration of Bacillus velezensis -fermented postbiotics for largemouth bass and demonstrated its important role in promoting host growth and enhancing disease resistance. These findings provide a theoretical foundation and strategic support for the application of postbiotics in antiviral control in aquaculture.