Bacillus velezensis NDB mitigates Aeromonas hydrophila-induced enteritis in black sea bream (Acanthopagrus schlegelii) by enhancing intestinal immunity and modulating gut microbiota

<i>Aeromonas hydrophila</i>-induced enteritis presents a significant challenge to the intensive aquaculture of black sea bream (<i>Acanthopagrus schlegelii</i>). Studies have shown that probiotic supplementation provides a promising alternative to conventional antibiotic applications for disease prevention in aquaculture. In this study, we investigated the protective effects of dietary supplementation with <i>Bacillus velezensis</i> NDB on the growth, immunity, and intestinal microbiota of black sea bream against <i>A. hydrophila</i>-induced intestinal damage. A total of one hundred and eight black sea bream fingerlings (initial body weight of 9.44 ± 0.03 g) were randomly assigned to three groups (total of nine tanks, twelve fish per tank): a control group (CON, normal diet), an infected group (AH, normal diet for 28 days followed by <i>A. hydrophila</i> infection at 1.0 × 10⁷ CFU/mL), and a probiotic-treated group (AH+NDB, diet supplemented with 1.43 × 10⁸ CFU/g <i>B. velezensis</i> NDB for 28 days, followed by <i>A. hydrophila</i> infection at 1.0 × 10⁷ CFU/mL). The results showed that the weight gain rate in the AH+NDB group was 139.44 ± 48.61%, which was significantly higher than that in the CN (126.81 ± 43.48%) and AH (132.48 ± 63.54%) groups. The pathological symptoms of black sea bream induced by <i>A. hydrophila</i>, including gill and abdominal hemorrhage, villus deformation, and inflammatory infiltration, were alleviated in the AH+NDB group. Histological and biochemical analyses showed the dietary supplementation of <i>B. velezensis</i> NDB enhanced antioxidant enzyme activities (SOD and CAT) and reduced lipid peroxidation (MDA) in the AH+NDB group. Compared to the AH group, the AH+NDB group exhibited significantly upregulated expression levels of anti-inflammatory markers (<i>il10</i> and <i>tgf-</i>β), and significantly downregulated levels of pro-inflammatory cytokines (<i>il1, tnf-</i>α, and <i>ifng</i>). Moreover, dietary supplementation with <i>B. velezensis</i> NDB increased the abundance of beneficial genera (e.g., <i>Bacillus</i> and <i>Ruegeria</i>), and decreased the abundance of opportunistic pathogenic genera (e.g., <i>Aeromonas</i> and <i>Vibrio</i>), thus enhancing the carbohydrate/amino acid biosynthesis and promoting the nucleoside and nucleotide biosynthesis to alleviate <i>A. hydrophila</i>-induced enteritis. Collectively, the study demonstrated that dietary supplementation with <i>B. velezensis</i> NDB can effectively promote growth performance and enhances immune function in black sea bream, thereby providing significant benefits for fish culture.