Isolation and Identification of Nocardia,A Pathogen of Nocardiosis in Largemouth Bass,Micropterus salmoides

Nocardiosis is a commercially important disease in cultured largemouth bass in China,causing substantial economic losses.However,the pathogen causing largemouth bass nocardiosis has not yet been isolated in China.In this study,a strain of Nocardia(designed as NH090627) was isolated from diseased largemouth bass reared in Nanhai,Guangdong.The bacteria appeared Gram-positive,bead-like or long,slender,filamentous rods in shapes.The traditional physiological and biochemical methods showed that most of the phenotypes of strain NH090627 were similar to Nocardia.Phylogeny analysis with 16S rDNA showed that the strain NH090627 was branched into the same cluster as that of other species of Nocardia,and had a higher identity with Nocardia seriolae JCM 3360T(99.9%).Based on morphological,physiological,biochemical properties and phylogenetic analysis,the filamentous rod bacterial was identified as Nocardia seriolea.Antimicrobial susceptibility assay showed that this strain was susceptible to chloramphenicol,streptomycin,kanamycin and tetracycline,but high resistant to sulphamethoxazole and ampicillin.