Characterization of probiotic Bacillus subtilis isolated from Nile tilapia ( Oreochromis niloticus ) digestive tract and evaluation its positive impact on health and nonspecific immunity of Nile tilapia

In the present study, a probiotic, Bacillus subtilis was isolated and identified from the digestive tract of freshwater Nile tilapia collected from production ponds of the Central Laboratory of Aquaculture Research, Egypt. The isolated strain of Bacillus subtilis was biochemically identified using API 20E and 16S rRNA. The biosafety assay confirmed that the isolate was not pathogenic to Nile tilapia fish. Isolated strain was able to survive at increasing concentrations of bile. The isolated strain was sensitive to most used antibiotics. The Isolated strain showed antagonistic activity against bacterial fish pathogens, including Aeromonas hydrophila and Pseudomonas aeruginosa. In addition, the isolated strain did not show any hemolytic activity against human blood in the blood agar medium. To evaluate the positive impact of the supplementation of an isolated strain of Bacillus subtilis on hematological, biochemical and immunological parameter of Nile tilapia. A total of 120 fish (20 g) were randomly divided into 2 groups (control and treatment with 3 replicates). The control groups were fed the basal diet. Treatment groups were supplemented with B. subtilis 1X10 cfu/g. Results showed that Nile tilapia fed with diets containing B.subtilis for 7 weeks had a significant increase in Hb%, RBCs count, HCT values and WBCs count. Also, there is a significant improvement in liver function test (ALT, AST, Total protein and Albumin). Also, there is a significant increase in immunological parameter (NBT, Lysozyme). Moreover, fish fed with B.subtilis supplemented diets showed improved survival rates after A. hydrophila infection. Egyptian Journal for Aquaculture P-ISSN: 2090-7877 E-ISSN: 2636-3984 www.eja.journals.ekb.eg ElAdawy M.M, 2019; 9 (2):01-26 DOI: 10.21608/eja.2019.13397.1000 El –Adawy M.M 2019; Egy. J. Aquac 9(2):01-26 2 Keyword: Bacillus subtilis , Nile tilapia , API 20E, Positive impact.