ABSTRACT
A feeding trial was conducted to investigate the effects of dietary administration of probiotic Bacillus subtilis and prebiotic fructooligosaccharide (FOS) on growth performance, immune responses and disease resistance of juvenile ovate pompano, Trachinotus ovatus. One thousand six hundred and twenty individuals (initial body weight: 10.32 ± 0.46 g, mean ± S.E) were fed nine practical diets according to a 3 × 3 factorial design: the basal diet as the control diet supplemented with three levels of B. subtilis (0, 1.05 × 10(7) or 5.62 × 10(7) CFU g(-1) diet), crossed with 0, 0.2% or 0.4% FOS. After an 8-week feeding experimental period, six fish per cage were sampled for immunity determination. Then 18 fish of each cage left were challenged by Vibrio vulnificus. The results showed that fish fed with 5.62 × 10(7) CFU B. subtilis g(-1) in combination with 0.2% FOS produced the highest specific growth rate, and were significantly higher than the groups fed with 0 and 0.2% FOS without B. subtilis supplementation (P < 0.05). Feed efficiency ratio significantly increased with the increasing doses of dietary FOS without B. subtilis added (P < 0.05). The immune assay showed that fish fed with the control diet produced the lowest respiratory burst activity and was significantly different from the groups fed the diets containing 0.2% FOS at each B. subtilis level and containing 0.4% FOS single (P < 0.05). Phagocytic activity was significantly decreased with the increasing doses of dietary B. subtilis at 0.4% FOS level (P < 0.05). Alternative complement pathway activity of the fish fed with 0.2% FOS single was significantly lower than those fed with 5.62 × 10(7) CFU B. subtilis g(-1) diet supplemented at each FOS level (P < 0.05). Fish fed with the control diet had the lowest lysozyme activity, and were significantly different from those fed with 0.2 or 0.4% FOS at 1.05 and 5.62 × 10(7) CFU B. subtilis g(-1) diet level. Moreover, fish fed with diets supplemented with 0.2% and 0.4% FOS at each B. subtilis level had notably lower cumulative mortality after 10 days following V. vulnificus infection (P < 0.05). Under the experimental conditions, dietary B. subtilis and FOS had a significant interaction on enhancing the immune responses and disease resistance of juvenile ovate pompano (P < 0.05).
