Synergistic effects of Bacillus pumilus and exogenous protease on Nile tilapia (Oreochromis niloticus) growth, gut microbes, immune response and gene expression fed plant protein diet

Abstract

The synergistic effects of probiotic (Bacillus pumilus) and/or exogenous enzyme (protease) on growth, immunity, serum parameters, gene expression and gut bacteria of Nile tilapia (3.62 ± 0.06 g) were investigated. Plant protein-based diets were formulated as the control diet, probiotic (pro, 1.85 × 105B. pumilus CFU kg−1), protease enzyme (enzy, 0.5 g protease kg −1), and their mixture of 1.85 × 105B. pumilus CFU kg−1 + 0.5 g protease kg −1 (pro-enzy), respectively. After 84 days, the results revealed that fish fed pro-enzy diet resulted in better (P < 0.05) growth performance and feed utilization, including highest goblet cells, thickness of musclaris, mucosal folds and enterocytes. In addition, the highest values of hematocrit (Htc), hemoglobin (Hb), red blood cells (RBCs), white blood cells (WBCs), immunoglobulin M (IgM), lysozyme and phagocytic and serum parameters, including albumin, globulin and total protein were detected in the pro-enzy treatment (P < 0.05). The best levels of serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were found in group fed pro-enzy diet. The highest relative levels of growth hormone (GH) were recorded in brain and liver of fish fed the control diet, while, insulin like growth factor (IGF-I) was higher in fish fed pro-enzy diet. Culture-independent molecular analyses of 16S ribosomal ribonucleic acid (16S rRNA) gene of gut bacteria showed that treatments directed intestinal bacteria towards health of fish, through inhibition of the growth of some hydrocarbon-degrading bacteria, such as Pseudomonas indica, and pathogenic bacteria, such as Citrobacter koseri. There were four genotypes, which homologized with mostly gas-fixating bacteria, were not affected by treatments, suggesting core gut bacteria. In conclusion, the addition of pro-enzy improved the growth performance, intestinal histological morphometric, immune response, hematological, biochemical blood, metabolic gene expression and intestinal bacterial flora of Nile tilapia fed diet free fish meal.