Studies On Resistance Of Nocardia Seriolae In Micropterus Salmoides And Pathological Morphology Of Micropterus Salmoides Infected By Nocardia Seriolae

Nocardia seriolae is an important pathogeny that seriously endangers the cultivation of Micropterus salmoides.At present,there are few studies on antimicrobial resistance characteristics,prevalence of resistance genes of N.seriolae and the pathomorphology of large mouth bass infected N.seriolae.Based on the isolation and identification of N.seriolae from the fish samples collected from the main large mouth bass culture areas in Sichuan Province,antimicrobial resistance characteristics,epidemiological study of resistance genes,pathological damage characteristics and pathogenic distribution were studied,to guide the rational use of antimicrobial agents in veterinary medicine and further clarify the pathogenic mechanism.From 2017 to 2018,69 samples were collected from several major large mouth bass culture areas in Sichuan Province.Through morphological,physiological and biochemical characteristics,16 S rRNA and hsp65 gene sequence analysis,20 strains of N.seriolae were isolated and identified.The broth microdilution chromogenic method was used to determine the antimicrobial resistance of the isolated strains to 18 antibiotics.The results showed that 20 strains were resistant to PEN,EFT,FOX,AMC and OXA,and the resistance rate was 100%.The resistance rates to ENR and OFX were 45% and 75% respectively.Resistance rates to SIZ and SXT were 40%,FLO was 20%,TIA was 35%,and 20 strains were sensitive to DOX,E,TIL,GEN,VA,LZD and DA.The results showed that tetracycline resistance genes tetA,tetB,tetC,tetD,tetE,tetG,tetK,tetL,tetM,tetO,tetQ and tetS were detected with 100%,20%,30%,85%,55%,100%,15%,10%,85%,30%,30%,5%,95%,respectively.The detection rates of ermB,ermC,mefA,msrA,mphA,lnuA,lnuB were 40%,35%,65%,15%,75%,50%,55%,respectively;the detection rates of blaSHV,blaTEM,blaVIM,blaZ,blaCMY,blaCMY2 and bla1 were 10%,100%,5%,15%,55%,60%,15%;the aminoglycoside resistance genes aac,AAC(6')-II,aacA/aphD,aacC,aacC2,80%,20%,20%,55%;The detection rates of catB3,catB8,catA1,cmlA1,cmx(A)and floR were 95%,95%,85%,20%,95% and 100%,respectively.The detection rates of acrA,acrB,acrF,mexA and mexF were 100%,100%,95%,5% and 100%,respectively.In this study,the antimicrobial resistance characteristics and resistance genes of N.seriolae were different apparently in different strain collected from different culture areas in Sichuan.The main gross pathological changes of diseased large mouth bass are surface ulcer,fin congestion,and nodules in liver,spleen,heart,head kidney,kidney and other visceral organs.Histopathologically,chronic granulomatous nodules appeared in liver,spleen,kidney,heart,gill and other tissues,accompanied by many pathological changes such as hemorrhage,degeneration,necrosis and inflammation.Granulomatous nodules are composed of caseous necrosis center,epithelioid cell layer and outermost fibrous connective tissue.Gram staining and electron microscopy showed that there were a large number of rod-shaped bacteria in the liver,spleen,kidney,heart and gill of large mouth bass,and most of them were located in the center of granuloma and the necrotic focus around it.Immunohistochemical staining showed that there were positive signals in liver,spleen,kidney,head and kidney,and most of them were located in and around the center of granuloma.In conclusion,N.seriolae could infect large mout bass.The bacteria could distribute in liver,spleen,kidney,head kidney,gill and other tissues and organs,and cause damage to these tissues and organs,and form obvious granulomatous nodules,which is of great significance in pathological diagnosis.