Effects of dietary probiotic on the growth performance, non-specific immunity and disease resistance of cobia, Rachycentron canadum

The present study was performed to investigate the effects of a commercially available probiotic product (compound probiotic) containing Bacillus subtilis 7.0 x 109 CFU g-1, Bacillus licheniformis 3.0 x 109 CFU g-1, Lactobacillus spp. 5.0 x 108 CFU g-1 and Arthrobacter spp. 1.0 x 108 CFU g-1 on the growth performance, non-specific immunity and protection against Vibrio harveyi infection in cobia (Rachycentron canadum). Fish were fed diets containing six graded levels of compound probiotic (0.0, 1.0, 2.0, 3.0, 4.0 and 5.0 g kg-1) for 8 weeks. The results showed that the survival rate ranged from 81.1% to 84.4% with no significant difference among dietary treatments (P > 0.05) after feeding experiment. Dietary compound probiotic significantly increased the specific growth rate (SGR), serum lysozyme, alternative complement pathway (ACP) activity, phagocytosis percentage (PP) and respiratory burst activity of head-kidney macrophages of cobia. Moreover, feeding of supplemented diets containing compound probiotic resulted in significantly lower mortality against the pathogens Vibrio harveyi compared with the control group. To elevate the growth and immune resistance ability of cobia, an optimal dose of dietary compound probiotic administration determined by second-order polynomial regression analysis was 3.3 g kg-1, on the basis of the SGR and mortality after challenge with V.harveyi.