Isolation,Identification and Pathological Analysis of Aeromonas caviae from Micropterus salmoides
[Objective]The study was conducted to investigate the causes of sudden death of largemouth bass(Micropterus salmoides)in a bass culture base in Shenzhen-Shanwei Special Cooperation Zone,and reveal the biological characteristics of the Pathogen,with an aim to provide theoretical reference for disease prevention,treatment and prevention in breeding largemouth bass.[Method]A dominant bacterial strain,named SZNH-S20230817,was isolated from the liver,spleen,intestine and lesion tissues of the diseased largemouth bass,and the isolate was subsequently subjected to morphological characteristics observation,physiological and biochemical characteristics identification,artificial infection experiment,drug sensitivity test,MIC and MBC determination,16S rRNA sequencing,identification of resistance genes and virulence genes to analyse its drug resistance and pathology.[Result]The results showed that the extended 16S rRNA sequence of SZNH-S20230817 had a 99.38%homology with Aeromonas caviae AP019195,and also carried four virulence genes,including aer,act,lip and fla.The results of physiological and biochemical identification showed that,indole,oxidase,phenylalanine,mannitol,xylose,arginine dihydrolysate and glucose acid production of the strain were positive,and the strain SZNH-S20230817 was identified as Aeromonas caviae.Regression infection revealed that the strain SZNH-S20230817 was highly pathogenic,with an LC50 of 1.39×107 CFU/mL for largemouth bass.The results of drug sensitivity tests showed that the bacterium was resistant to 12 antibiotics such as cefazolin,kanamycin,tetracycline and vancomycin,ampicillin,and clindamycin,and also carried 13 resistance genes such as qnrS,catI,emrB,Sul1,rmtB and etc.Histopathological analysis showed that infection with the strain SZNH-S20230817 could cause swelling and freeing of splenocytes,fracture and rupture of intestinal mucosa,shedding of villi,necrosis of renal cells,infiltration of inflammatory cells and vacuolisation of hepatic cells in the fish.The results of in vitro bacterial inhibition tests showed that the bacterium had low MIC and MBC against rheum officinale and gallnut,and the remaining seven herbs had weak inhibition against the bacterium.[Conclusion]Aeromonas caviae is the pathogen that causes the sudden death of largemouth bass,which carries four virulence genes of aer,act,lip and fla,and has an LC50 of 1.39×107 CFU/mL against largemouth bass.It is resistant to antibiotics such as cefazolin,kanamycin and tetracycline.Low dose of rheum officinale and gallnut can inhibit the growth of the bacterium effectively,with good preventive and inhibitory effects.
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