Abstract
Intensive aquaculture practices heighten oxidative stress and infectious disease risk, necessitating sustainable alternatives to antibiotics. This study evaluated the integrative probiotic and postbiotic potential of Bacillus velezensis AAHM-BV2302 in red tilapia (Oreochromis spp.), focusing on growth, antioxidant defense, immune modulation, and resistance to Streptococcus agalactiae. Whole-genome sequencing confirmed its classification as B. velezensis (4.16 Mb, GC 45.9%, ANI 99.4% with NRRL B-41580). Fish were fed diets supplemented with probiotic cells (Cell), cell-free supernatant (Cfs), or their combination (Cell + Cfs) for 30 days, followed by 30 days without probiotic supplementation. Growth performance significantly improved in Cell and Cell + Cfs groups at both Day 30 and Day 60 (p < 0.05). Antioxidant enzymes (SOD, CAT, GSH) increased significantly across tissues at Day 30, while malondialdehyde (MDA) declined (p < 0.05), indicating enhanced redox homeostasis. Humoral immunity was elevated, with higher lysozyme, bactericidal activity, and total IgM persisting post-supplementation (p < 0.05). Expression of il1b, il6, and il8 was upregulated in immune-related and mucosal tissues, reflecting robust immune activation (p < 0.05). After S. agalactiae challenge, survival rates were 55% in Cfs, 60% in Cell, and 70% in Cell + Cfs, corresponding to relative percent survivals (RPS) of 43.8%, 50.0%, and 62.5%, respectively (p < 0.05). These results demonstrate that B. velezensis AAHM-BV2302 enhances growth, antioxidant capacity, and immune resilience through complementary probiotic-postbiotic mechanisms, supporting its application as a safe, multifunctional biotic for antibiotic-free tilapia aquaculture.