Effects of dietary supplementation of Bacillus subtilis and fructooligosaccharide on growth performance, survival, non-specific immune response and disease resistance of juvenile large yellow croaker, Larimichthys crocea

A feeding experiment was conducted to examine the effects of dietary administration of Bacillus subtilis and fructooligosaccharide (FOS) on growth performance, survival, immune responses and disease resistance of juvenile large yellow croaker, Larimichthys crocea (mean initial body weight 7.82 g ± 0.68). Nine practical diets were formulated to contain three levels of B. subtilis (0.0, 0.42 × 107 cfu g−1 and 1.35 × 107 cfu g−1), each with three FOS levels (0, 0.2% and 0.4% of dry weight). Each diet was randomly assigned to triplicate groups of 60 juveniles. The experiment was conducted in floating sea cages (1.0 × 1.0 × 1.5 m) for 10 weeks. At the termination of the feeding trial, alternative complement pathway (ACP), superoxide dismutase (SOD) and lysozyme activity of serum and respiratory burst activity of head kidney macrophage were determined and fishes were challenged intraperitoneally with Vibrio harveyi. The results showed that at each dietary FOS level, dietary supplementation of 1.35 × 107 cfu g−1B. subtilis significantly increased the specific growth rate (SGR) (P < 0.01) and feed efficiency ratio (FER) (P < 0.05) compared with the groups without B. subtilis supplementation. The immune assay showed that at each FOS level, compared to the groups without B. subtilis supplementation significantly enhanced serum lysozyme was observed in fish fed the B. subtilis-supplemented diets (P < 0.05) and significantly enhanced serum SOD activity was observed in fish fed the diet with 1.35 × 107 cfu g−1B. subtilis (P < 0.05), while the serum ACP activity and the respiratory burst activity of head kidney macrophage were independent of dietary treatments. The challenge experiment showed that compared to the groups without B. subtilis supplementation the cumulative mortality after infection with V. harveyi was significantly lower in fish fed the diet with 1.35 × 107 cfu g−1B. subtilis (P < 0.05), at each FOS level. However, at each dietary B. subtilis level, addition of FOS in diets did not significantly affect the growth performance, immune response and disease resistance of large yellow croaker. No significant interactions were observed between dietary B. subtilis and FOS. These results showed that dietary supplementation of B. subtilis at a dose of 1.35 × 107 cfu g−1 improved growth, feed efficiency ratio, non-specific immune responses and disease resistance of juvenile large yellow croaker, L. crocea.