Isolation,Identification and Pathological Analysis of Vibrio alginolyticus from Epinephelus coioides
[Objective]In October 2023,skin ulceration,subcutaneous hemorrhage and enteritis appeared in a breeding base of Epinephelus coioides in Shenzhen,China.We clarified the causative organisms that triggered the sudden death of E.coioides and their biological characteristics,and screened the disinfectant that effectively inhibited the causative organisms,with an aim to provide theoretical guidance for the scientific prevention and control of E.coioides infected with Vibrio alginolyticus.[Method]The pathogen of V.alginolyticus was isolated from the diseased tissues of the dying E.coioides by streak plate method,the morphological characteristics,physiological and biochemical properties,phylogenetic tree,virulence genes and resistance genes as well as drug sensitivity of the pathogen were investigated,and regression infection tests were carried out.[Result]A dominant strain was isolated from the diseased tissues of E.coioides and named as strain SZGP226.The results of phylogenetic analysis showed that,the sequence of strain SZGP226 was homologous to V.alginolyticus HH101307,IB1 and Xmb015,with a similarity of 99.72%.The strain was identified as positive in physiological and biochemical indexes such as sucrose,mannitol,glucose,urea,oxidase activity and V-P,and it also carried five virulence genes,namely,tlh,toxR,ctsA,sto and vpi.The results of drug sensitivity showed that strain SZGP226 was resistant to 9 antibiotics like ampicillin and benzoxacillin,and carried 9 resistance genes including qnrB,floR,tetC,tetM,qnrD,gyrA,Sul1,SHV,and Oxa-24.The screening results of disinfectants showed that,sodium hydroxide had the best bactericidal effect,and its minimal inhibition concentration(MIC)was 8 mg/mL.The regression infection test showed that strain SZGP226 was strongly pathogenic to the E.coioides,with a LC50 of 1.63×107 CFU/mL,and it could cause obvious pathological damage to the liver,kidney and spleen of the E.coioides.[Conclusion]V.alginolyticus was the main pathogenic bacterium for the sudden death of the E.coioides,and its causative genes included tlh,toxR,ctsA,stoandvpi,and the LC50 was 1.63×107 CFU/mL.Sodium hydroxide(3 mg/mL)showing the best bactericidal effect on strain bactericidal effect could be selected to clean up the culture environment before aquaculture was carried out.
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