Dietary Bacillus velezensis S141 enhances immunity and resistance to Aeromonas veronii in Nile tilapia: insights from early transcriptomic and functional analyses.

Multifunctional probiotics offer a promising strategy for sustainable aquaculture. This study evaluated Bacillus velezensis S141 by integrating in vitro and in vivo approaches to assess its effects on growth and immune responses in Nile tilapia. The strain produced hydrolytic enzymes (amylase, protease, and lipase), demonstrated tolerance to bile salts across a wide pH range and temperature spectrum, and exhibited antagonistic activity against Aeromonas veronii, Flavobacterium columnare, and Streptococcus iniae. It also showed catalase activity, non-hemolytic properties, and antibiotic susceptibility. A 90-day feeding trial was performed using Nile tilapia fed diets containing 0, 106, and 108 CFU/g B. velezensis S141. Supplementation at 106 and 108 CFU/g improved growth performance at days 60 and 90. Immune parameters (ACH50, total Ig, phagocytic activity) were significantly elevated at 108 CFU/g at days 30 and 90, whereas antioxidant activities (SOD, CAT, MDA, and TAC) were markedly improved at 108 CFU/g at day 90. Fish were challenged with A. veronii on days 30 and 90 to evaluate protective efficacy. The day 30 challenge revealed early immune priming, with upregulation of proinflammatory cytokines (on-cc1, on-cxc1, and il-1β) within 6-12 h, whereas tnfβ peaked at 48 h. Transcriptomic analysis revealed enrichment of immune-related pathways, including cytokine-cytokine receptor interaction and NOD-like receptor signaling, indicating enhanced antibacterial defense. On day 90, probiotic-treated groups exhibited higher survival, demonstrating sustained protection. Overall, B. velezensis S141 improved growth, immune response, antioxidant status, and resistance to A. veronii in Nile tilapia.