Edwardsiellosis, caused by Edwardsiella tarda, poses a significant threat to the aquaculture industry, particularly in pangasius farming. This study investigates the effects of probiotic Bacillus subtilis AAHM-BS2360 and its postbiotic metabolites on growth performance, immune responses, antioxidative activity, and disease resistance against E. tarda infection. A total of 240 healthy pangasius (37.0 ± 4.9 g) were divided into four treatment groups with four replicate tanks each, as follows: (1) the Control group, which received feed top-dressed with 100 mL of 0.85% NaCl/kg diet; (2) the Probiotic group, which received feed supplemented with 100 mL of B. subtilis AAHM-BS2360 cells at the concentration of 1 × 10¹² CFU/kg diet; (3) the Postbiotic group, which received feed supplemented with B. subtilis AAHM-BS2360 cell-free supernatant 100 mL/kg diet; and (4) the Pro + Post group, which received a combination of B. subtilis AAHM-BS2360 cells and cell-free supernatant. After 30 days of feeding treatment, biochemical serum analysis revealed a significant increase in the AST/ALT ratio in the Postbiotic group. The Probiotic and Postbiotic treatments increased lysozyme activity in mucus, indicating an innate immune response to pathogens. The Pro + Post group exhibited the highest levels of catalase (CAT) in serum and upregulated antioxidant-related genes. All treatment groups receiving B. subtilis AAHM-BS2360, metabolites, and their combinations showed significant upregulation of immune-related genes, like lygl1, tgfb, b2ml, and tnf. The expression of proinflammatory genes (litaf, ifngr1l, c3, il13, and il1b) increased, with the most pronounced effects observed in the Pro + Post group. The Probiotic group showed significant upregulation of the growth-related gene igf1. Meanwhile, the Pro + Post group showed significantly higher values in SGR and ADG parameters, with values of 3.29 ± 0.98%/day and 1.42 ± 0.52 g/day respectively (p < 0.05). Survival rates were significantly higher in the Pro + Post (87.5%), Postbiotic (84.37%), and Probiotic (81.25%) groups when challenged with E. tarda. Dietary supplementation with B. subtilis AAHM-BS2360, its metabolites, and their combination enhanced immune response, reduced oxidative stress, and improved growth performance in pangasius, highlighting its potential as a functional feed additive for sustainable aquaculture.

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.

This study investigated the immunonutritional potential of high-molecular-weight (Mw~85 kDa), non-degraded chitosan (NCS) and gamma-radiation-degraded, low-molecular-weight chitosan (RCS) incorporated into aquafeeds for Nile tilapia (Oreochromis niloticus). RCS was produced by γ-irradiation (10 kGy) in the presence of 0.25% (w/v) H₂O₂, yielding low-viscosity, colloidally stable nanoparticles with Mw ranging from 10 to 13 kDa. Five diets were formulated: a control, NCS at 0.50%, and RCS at 0.025%, 0.050%, and 0.075%. No adverse effects on growth were observed, confirming safety. Immune gene expression (e.g., ifng1, nfκb, tnf), antioxidant markers (e.g., reduced MDA, increased GSH and GR), and nonspecific humoral responses (lysozyme, IgM, and bactericidal activity) were significantly enhanced in the NCS-0.50, RCS-0.050, and RCS-0.075 groups. Notably, these benefits were achieved with RCS at 10-fold lower concentrations than NCS. Following challenge with Edwardsiella tarda, fish fed RCS-0.050 and RCS-0.075 diets exhibited the highest survival rates and relative percent survival, highlighting robust activation of innate and adaptive immunity alongside redox defense. These results support the use of low-Mw RCS as a biologically potent, cost-effective alternative to traditional high-Mw chitosan in functional aquafeeds. RCS-0.050 and RCS-0.075 show strong potential as immunonutritional agents to enhance fish health and disease resistance in aquaculture.