Probiotics can be used as immunostimulants in aquaculture. The aim of this study was to evaluate the immune responses of Nile tilapia Oreochromis niloticus following feeding with Bacillus amyloliquefaciens spores at concentrations of 1 × 10(6) (G3) and 1 × 10(4) (G2) colony-forming units per gram (CFU/g) of feed compared with a basal diet with no probiotics (G1). A total of 180 fingerlings (27.7 ± 0.22 g) were divided into three groups (G1-G3 of 20 fish per group) in triplicate. Innate immunities were measured every two weeks based on serum bactericidal activity, lysozyme activity, a nitric oxide assay (mmo/l) and phagocytic activity, and the expressions of interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF α) were examined after one month. Moreover, the survival of tilapia upon challenge with Yersinia ruckeri or Clostridium perfringens type D was determined at the end of feeding trial. After 15 d, the serum killing percentages and phagocytic activities were significantly higher in G3 than in G1 and G2, whereas the same parameters had significantly higher values in G3 and G2 than in G1 after 30 d. After both 15 d and 30 d, the lysozyme activities and nitric oxide assay results (mmo/l) were significantly higher in G3 than G2, and the lowest values were observed in G1. The percentage of serum killing, serum nitric oxide and serum lysozyme activity were significantly increased by the time of B. amyloliquefaciens administration independently of the probiotic dose, and the phagocytic activity percentage was significantly decreased at the end of the experiment. Dietary B. amyloliquefaciens caused significant increases in IL-1 and TNF α mRNA levels in the kidneys in the following pattern: G3 > G2 > G1. Fish that were fed B. amyloliquefaciens exhibited better relative survival percentages than the controls when challenged by Y. ruckeri or C. perfringens type D. Dietary supplementation with B. amyloliquefaciens improves immune status and disease resistance in Nile tilapia.
Keywords: Bacillus amyloliquefaciens; Clostridium perfringens; Immunity; Nile tilapia; Yersinia ruckeri.
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