Probiotic Bacillus licheniformis competitively establishes and enhances production efficiency, immune responsiveness, and protection against Aeromonas hydrophila O:18 in Cirrhinus mrigala (Ham.).

The use of probiotics is increasingly becoming a strategic tool in aquaculture production systems to mitigate the impacts of antimicrobial resistance. The effects of dietary Bacillus licheniformis on the immune response and disease resistance of Cirrhinus mrigala were investigated following a 30-day growth trial. One hundred eighty fingerlings (10 ± 2 g) were randomly assigned to five groups (three replicates each) and fed either a control diet or diets supplemented with B. licheniformis at 1.0 × 10⁶ cfu g⁻1 (BL-6), 0.5 × 10⁷ cfu g⁻1 (BL-05.7), 1.0 × 10⁷ cfu g⁻1 (BL-7), or 0.5 × 10⁸ cfu g⁻1 (BL-8). Probiotic supplementation significantly (P < 0.05) improved weight gain percentage, specific growth rate, and feed conversion ratio. B. licheniformis at 10⁷ cfu g⁻1 and higher competitively replaced and decreased Gram-negative rods (from >75% to <45%), specifically Aeromonas (60 → 40), Pseudomonas (10 → <2), Alcaligenes (6 → 2), and Vibrio (4 → 0) at the genus level, with a concomitant increase in Gram-positive bacteria (25% → 55%), particularly Bacillus (3 → >40), in the intestine of C. mrigala. B. licheniformis supplementation increased total erythrocyte and leukocyte counts, hemoglobin, and serum protein, albumin, and globulin levels, while serum alkaline phosphatase (ALP), alanine aminotransferase (ALT), and aspartate aminotransferase (AST) activities decreased (P < 0.05). At or above 10⁷ cfu g⁻1, B. licheniformis significantly improved both cellular immune responses (phagocytic index and ratio, respiratory burst activity, serum myeloperoxidase activity, ALP, and total leukocyte counts) and humoral (including adaptive) immune responses (lysozyme activity, serum bactericidal activity, serum haemagglutination titre, and natural haemolysin titre), as well as survival following challenge with A. hydrophila O:18. These findings support the potential use of this bacterial species as a probiotic candidate in diets for the culture of C. mrigala.