Isolation and identification of Bacillus sp.and evaluation of its effect on WSSV disease resistance in Litopenaeus vannamei

In order to select WSSV disease-resistant strains,a marine Bacillussp.was isolated and purified from digestive tract from the healthy of Chinese shrimp(Fenneropenaeus chinensis).The morphology and gram stain indicated that the strain is a gram-positive and rod-shaped bacterium,with a single polar flagellum and oval spores.The colony is circular and slightly raised.Identification analyses by the Biolog Carbon Source Utilization,ATB Microbial Identification System,and the fatty acid gas chromatography indicated that the most similar strain in physiological and biochemical characteristics is Bacillus firmus.Phylogenetic analysis with 16S rRNA sequence showed that it has 100% homology with the previously reported Bacillus firmus.The cultured strain PC024 was added to feed by conglutinating to the surface of the pellets and fed to Litopenaeus vannamei.After feeding for 20 d,the shrimp was challenged with WSSV by intramuscular injection to observe the cumulative mortality in 14 d post-challenge.The results showed that the experimental group fed with the strain PC024 had a relative survival rate of 33.7% in comparison with the control group.The immune-related enzyme activity in the serum and hepatopancreas of shrimp in the experimental group was significantly increased than the control group.And the total number of bacteria of the intestine of the experimental group is always significantly higher than that of the control group and Bacillus firmus can be isolated from the experimental group.This study suggests that the Bacillus firmusPC024 can be used as the WSSV disease prevention probiotic strains and can further be used in shrimp farming.