Influence of dietary Bacillus amyloliquefaciens AV5 on growth, immune responses, and resistance to Streptococcus agalactiae in Nile tilapia (Oreochromis niloticus)

This study assessed the effect of Bacillus amyloliquefaciens AV5 on the diet of Nile tilapia, focusing on growth, immune responses, and resistance to Streptococcus agalactiae infection. Fingerlings of Oreochromis niloticus were split into three groups and fed diets with different concentrations of B. amyloliquefaciens AV5 for 30 days as follows: no AV5, G1:106 CFU/g, G2:108 CFU/g. Upon completion of the feeding trial, G2 exhibited improved growth performance, including Wf, WGR, and SGR, along with a significantly lower (p< 0.05) feed conversion ratio (FCR) than the other groups. Enhanced immune and biochemical parameters (p < 0.05), including lysozyme (LZM), immunoglobulin M (IgM), alkaline phosphatase (AKP), and acid phosphatase (ACP), as well as antioxidant enzymes (superoxide dismutase (SOD) and catalase (CAT)) and digestive enzymes (trypsin (TYS), lipase (LPS), and protease (PTS)), were observed in group G2 compared to the GC group. No Significant differences in amylase (AML), cellulase (CLS), and xylanase (XYN) levels were observed among the GC, G1, and G2 groups. A substantial decrease (p< 0.05) in GOT, GPT, and MDA activity was observed in G2 compared to that in the GC group. No substantial differences in glucose (GL) levels were observed among the GC, G1, and G2 groups. A substantial increase (p< 0.05) in total protein (TP) was observed in G2 relative to the GC and G1 groups. Significant increases (p< 0.05) were observed in the liver and kidneys of growth- and immunity-related genes in the G2 group, including C-LYZ, GHR-1, TNF-α, IL-1β, and IGF-1. After intraperitoneal injection of S. agalactiae into both control and test fish, the cumulative mortalities in the GC, G1, and G2 groups were 78 %, 51 %, and 14 %, respectively. The findings indicate that administering B. amyloliquefaciens AV5 at a concentration of 1×10⁸cfu/g could potentially yield beneficial outcomes as a probiotic in aquaculture.