Isolation and Characterization of Aeromonas veronii from Ornamental Fish Species in China

Aeromonas veronii infection of fish has become a significant problem in aquaculture. In this study, pure bacterial cultures were isolated from diseased ornamental fish suffering high mortality rate in China. These fish included zebrafish (Danio rerio), goldfish (Carassius auratus), oscar (Astronotus ocellatus), parrotfish (Amphilophus spp.) and crucian carp (Cyprinus carpio koi). Using morphologic, physiological, and biochemical characteristics and 16S rRNA sequences analysis, nine isolates, namely strains ZG-1, ZG-2, DW-1, DW-2, KL-1, DT-116, LS-912, SJ-1 and SJ-2 were identified as A. veronii. A phylogenetic tree constructed on the basis of 16S rRNA sequences strongly indicated that the isolates were most closely related to the species A. veronii (98.43-100% similarities). The pathogenicity of the isolate was confirmed in zebrafish with median lethal dose (LD50) ranging between 1.15×106 CFU/mL and 3.94×108 CFU/mL. Antibiotic susceptibility tests showed that the isolated strains were all susceptible to cephalosporins and chloramphenicol. Furthermore, histopathology of zebrafish showed leukocyte infiltration of gill filaments, necrosis of the hepatic cell and glomerulus hemorrhage. These results will provide a scientific reference for the prevention, control, and understanding of the pathogenic mechanism of A. veronii infection in fish.