Synergistic Effects of Silica Nanoparticles, Chitosan and Bacillus velezensis AAHM-BV2301 on the Growth, Immunity, Gut Microbiota and Disease Resistance of Asian Seabass (Lates calcarifer)

In this study, the synergistic effects of dietary <i>Bacillus velezensis</i> AAHM-BV2301, silica nanoparticles (SiNPs), and chitosan (CS) on the growth performance, innate immunity, gut microbiota, and disease resistance of Asian seabass (<i>Lates calcarifer</i>) fingerlings were evaluated. A total of 400 fish (11.25 ± 2.12 g) were assigned to five dietary treatments for 30 days: control, BV (1 × 10⁸ CFU/kg feed), BVSiNP (1 × 10⁸ CFU/kg + 2 mg SiNP/kg), BVCS (1 × 10⁸ CFU/kg + 15 g CS/kg), and BVSiNPCS (combined additives at the same concentrations). The growth indices (WG, SGR, RGR, and FCR) significantly increased in the fish fed BVSiNPs, whereas the level of innate immunity increased across all the supplemented groups, with BVCS and BVSiNPCS having the strongest respiratory burst and lysozyme activities. The tissue-specific modulation of immune-related genes (<i>α2M, HSP70</i>, <i>Mx</i>, and <i>C3</i>) was most pronounced in BVSiNP-fed fish, particularly in the gills and liver. Gut microbiome profiling revealed enrichment of <i>Cetobacterium somerae</i> in response to BV-based treatments, whereas BVSiNPCS induced the greatest increase in microbial richness and network connectivity. Postchallenge survival against <i>Vibrio vulnificus</i> was significantly greater in the BV and BVSiNP groups (<i>p</i> < 0.05). Overall, SiNPs acted as functional enhancers of the <i>B. velezensis</i> probiotic, supporting improved growth, immune activation, and microbiota restructuring. These results highlight the potential of nanoparticle-integrated synbiotics for microbiome-targeted health management in aquaculture.