Comparative on characteristics and pathogenicity of Nocardia seriolae isoalted from 9 fishes

Abstract:

[Background] Nocardia seriolae is a kind of Gram-positive aerobic bacterium, which may cause sarcoidosis in both freshwater and marine fish, among them, perciformes are the most susceptible to the disease. In recent years, serious losses caused by N. seriolae in freshwater fishes such as Micropterus salmoides and Channa argus have been increasing. [Objective] To compare and analyze the homology, growth characteristics, pathogenicity and antimicrobial susceptibilities, 9 N. seriolae strains isolated from different freshwater fish species suffering from sarcoidosis in different years and regions were determined in this study, and to further develop a reliable scientific measures to control and treat this disease. [Methods] 16S rRNA gene and the housekeeping gene secA1 were amplified and sequenced for identification and phylogenetic analysis of N. seriolae after biological characteristics and phylogenetic analysis. PCR-restriction fragment length polymorphism (PCR-RFLP) was developed to analyze the homoology of different origins of N. seriolae. Growth curve of 9 strains was determined by in vitro culture. Pathogenicity of 9 N. seriolae strains was characterized by artificial injection against largemouth bass. Antimicrobial susceptibility testing was carried out to determine the phenotypes of 13 antimiocribal agents by microbroth double dilution method. Virulence genes and drug resistance genes of 9 strains were also detected in this study. [Results] Based on the biological characteristics, most of the phenotypes in 9 strains were similar to Nocardia seriolae. Nine strains were identified to be N. seriolae and clustered into one category with high intraspecies similarity. PCR-RFLP digestion patterns were identical and the growth curvre of 9 strains showed no significant difference of growth characteristics. A variety of virulence genes were detcted in 9 strains. Three genes of mce1A, mig and pup were all detected in 9 strains, whereas dop and whiB3 genes were only carried in some of the strains. There was no significant difference in the pathogenicity of 9 N. seriolae to largemouth bass. All the strains were resistant to sulfamonomethoxine, flumequine and ampicillin, while most of the strains were susceptible to other antimicrobial agents tested. bla_TEM gene was detected in 9 N. seriolae strains, some of which also carried sul1 gene. [Conclusion] Nine strains of N. seriolae isolated from diferent origins showed similar growth characteristics, pathogenicity and antimicrobial susceptibilities. It implied that N. seriolae was the predominant pathogen of sarcoidosis prevailed in freshwater fish and with similar homology. The results of the study provide foundation of further research on the pathogenesis, prevention and control technology of N. seriolae.