Abstract:Aeromonas is widely distributed in freshwater environments.Since the late 1980s, Aeromonas has caused serious economic losses to the aquaculture industry in our country.The published species names are increasing in the genus Aeromonas.In order to determine the disease-causing bacterium at one fish farm of Nanjing, some tissue samples from diseased fish were collected and the pathogenic bacteria were identified by means of bacterial culture, biochemical characterizations and gyrB gene sequencing.One strain of bacterium was isolated and named LK-25.The results of biochemical tests showed that the bacterial strain has many similarities compared with A.hydrophila.Then the strain was subjected to sequencing of the gyrB gene for species identification, and then it demonstrated that the strain shared 99% homology with the A.aquariorum strain MDC 47.The corresponding gyrB phylogenetic analysis including representative strains from some known Aeromonas species showed that A.aquariorum LK-25 was very closely related to the A.hydrophila ssp.dhakensis.The detection result of virulence factors indicated that A.aquariorum LK-25 strain carried five virulent genes, including act, ahp, epr, hcp and alt genes, which encoded the cytotoxin, serine proteases, extracellular protease, hemolysin co-mediate protein and cytotonic enterotoxins, respectively.A.aquariorum LK-25 also showed strong hemolytic and proteolytic activities.The median lethal dose(LD50)with 1.02×103 CFU/ml in zebrafish indicated that A.aquariorum LK-25 is a highly virulent strain.A.aquariorum was a new species originally isolated from ornamental fish in 2008, and widely distributed in clinical and environmental specimens.Recent reports highlighted the importance of this species in cases of gastroenteritis, soft tissue infections, bacteremia and peritoneal dialysates in human, suggesting its public health significance.To our knowledge, this was the first report of A.aquariorum in China.The study will lay a good foundation for a better understanding of the occurrence and spread of A.aquariorum infection.