Analysis Of Biological Characteristics And Pathogenicity Of Nocardia Seriolae From 9 Fishes

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,the reports of Micropterus salmoides and Channa maculata ♀× Channa argus ♂ infected by N.seriolae in Guangdong province have been increasing.The researches of N.seriolae are mainly focuses on bacterial isolation and identification,rapid detection,and vaccine development,however,there are little informations of researches on epidemiological characteristics,pathogenic characteristics and pathogenic mechanism in N.seriolae.To find out the homology and biological characteristics of N.seriolae from different origins,the pathogenicity and tissue tropism of 4 species of fishes after infected,the physiological and biochemical characteristics,genotypes,virulence factors,and drug resistance of N.seriolae from different origins of fishes were compared in this research,the pathogenicity differences and the colonization trend in tissues infected by N.seriolae in 2 fishes were also analyzed in this study.The results of biological characteristics indicated that the phenotypes of 9 strains were similar to the species of N.seriolae.Nine strains were identified to be N.seriolae and were 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 detected in 9 strains,three genes of mcelA,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 M.salmoides.All the strains were resistant to sulfamonomethoxine,flumequine and ampicillin,while most of the strains were susceptible to other antimicrobial agents tested.blaTEM gene were detected in 9 N.seriolae strains,some of which also carried sull gene.It suggested that the N.seriolae prevailed in South China might be the same epidemic strain.There was no significant difference in the pathogenicity of N.seriolae to M.salmoides and C.maculata ♀× C.argus ♂,white nodules and granul-omatous structure of tissue were found in died fish.However,Oreochromis niloticus and Ctenopharyngodon idella were not both infected by the N.seriolae.The qPCR results showed that the copies of N.seriolae in the kidney of C.maculata ♀× C.argus ♂could reach up to 1.48 ×109 copies/g in the post stage of infection..In the early stage of infection(within 1-3 d),the bacterial copies in all tissues increased,and then maintained in a stable level.In M.salmoides,the copies of N.seriolae in kidney could reach up to 1.0×1010 copies/g in the post stage of infection.no changes were determined in blood.while there was a decrease in the early stage of infection in liver,heart,kidney and spleen,and then increase after 7 d.It indicated that M.salmoides and C.maculata ♀×C.argus ♂ were susceptible to N.seriolae,heart,liver,spleen and kidney were the target organs of N.seriolae.In conclusion,nine strains of N.seriolae isolated from fishes in South China showed similar homology,biological characteristics and pathogenicity.M.salmoides and C.maculata ♀× C.argus ♂ are susceptible to N.seriolae which cause systemic infection and high mortality after infected.The results of the study provide foundation of further research on the prevention and control of nocardiasis.