Isolation, identification and biological characteristics of Aeromonas veronii from Cyprinus carpio

【Objective】 Pathogenic Aeromonas veronii for carp was isolated and identified,to study the causes of death in cultured common carp(Cyprinus carpio) subjected to disease outbreaks,with the aim of providing references for the prevention and control of economic fish diseases in aquaculture.【Method】 The pathogenic bacteria were isolated from the visceral tissues of the diseased common carp and the taxonomic status was investigated by using morphological observation,physiological and biochemical identification,and analysis on the gene sequences of 16S rDNA and gyrB. Artificial regression infection experiments were used to assess the pathogenicity and antibiotic susceptibility tests were conducted to investigate the resistances to antibiotics.【Result】 The bacterial pathogen LY623 isolated from the viscera of the diseased carp was A. veronii according to the results of morphological observation and physiological and biochemical experiments. The 16S rDNA and gyrB gene sequences analysis showed that LY623 strain had the highest similarity with A. veronii strains and was clustered together with the strains of A. veronii based on the phylogenetic tree.Analysis results of virulence genesand drug resistance genes showed that the LY623 strain carried five virulence genes including Alt, Act, hlyA, lip and ela and five drug resistance genes including tem, ctxM, tetC, qnrA and qnrS. The results of antibiotic susceptibility test showed that LY623 strain was resistant to fourteen antibiotics such as tetracycline, norfloxacin,and erythromycin,and highly susceptible to fourteen antibiotics such as gentamicin,florfenicol,and chloramphenicol. The results of artificial regression infection experiments showed that the median lethal dosages(LD₅₀) of this strain were 2.64×10³ CFU/g and 4.77×10⁷ CFU/g to C. carpio and Danio rerio,respectively.【Conclusion】 The bacterial pathogen LY623 isolated from the visceral tissues of diseased common carp is A. veronii. It carries five virulence genes including Alt, Act, hlyA, lip and ela and five drug resistance genes including tem, ctxM, tetC, qnrA and qnrS. It has high virulence to the common carp,causing obvious histopathological changes in the tissues such as intestine,gill,liver,and spleen. Gentamicin,florfenicol,and chloramphenicolare alternative antibiotics to control the common carp diseases caused by A. veronii infection.