Modulation of growth performance, non-specific immunity, intestinal morphology, the response to hypoxia stress and resistance to Aeromonas hydrophila of grass carp (Ctenopharyngodon idella) by dietary supplementation of a multi-strain probiotic

The present study was conducted to evaluate a multi-strain probiotic (MP) on growth performance, immune and antioxidant function, response to hypoxia stress and resistance to Aeromonas hydrophila of grass carp ( Ctenopharyngodon idella ). Based on the viable cell counts of aerobic Bacillus spp., six experimental diets with MP supplemented at 0, 0.34, 1.68, 3.36, 6.72, 10.1 g kg −1 were formulated and 900 juveniles (7.30 ± 0.01 g) were equally distributed into 30 aquaria with respective diet for 60 days. Results showed that fish with 0.34–1.68 g kg −1 MP had better growth and feed utilization. Further, plasma total protein, albumin and high-density lipoprotein were remarkably increased with dietary MP at >1.68 g kg −1 . Dietary MP supplementation at 6.72–10.1 g kg −1 strikingly elevated plasma myeloperoxidase activity and complement C3 content. For fish with MP at 1.68 and 6.72–10.1 g kg −1 , their liver malondialdehyde and glutathione peroxidase were remarkably declined and promoted. After hypoxia stress, fish with 3.36–6.72 g kg −1 MP showed significantly higher respiratory burst activity. Challenge test by A. hydrophila confirmed the protection effects of MP through the decreased cumulative mortality rates. For intestinal histomorphology and enzymatic analyses, fish with 1.68 g kg −1 MP displayed significantly higher intestinal villi height, goblet cells and alkaline phosphatase activity. In conclusion, dietary MP supplementation at 1.68 g kg −1 could promote growth, intestinal morphology and antioxidant capacity , while enhancing host immunity requires higher dosages of MP. Broken-line analysis of weight gain revealed that 1.34 g kg −1 is the optimum dosage for the growth of grass carp.