Evaluation of Virulence Genes (aerolysin, elastase, lipase) in the Pathogenesis of Aeromonas hydrophila in Common Carp (Cyprinus carpio)

Aeromonas hydrophila is the causative agent of hemorrhagic septicemia in freshwater fish, especially in carp. Several factors are involved in the pathogenesis of the disease by this bacterium but the mechanism is yet to be investigated. Virulence genes are the most important pathogenicity factor in A. hydrophila. In this study, 30 isolates were identified as A. hydrophila by common biochemical methods and detection of 16S rDNA molecular marker using PCR method. PCR were then used to identify Aerolysin (aerA), Elastase (ahyB) and Lipase (lip) virulence genes using specific primers for each gene. The highest frequency belonged to the elastase gene (70%) and the aerolysin gene had the lowest abundance with (53.3%). For these three virulence genes eight different profiles were considered that the lipﶨ爇窰 isolates (33.3%) was the most frequent profile and lipﶨ/aerA (3.3%) was the lowest frequency obtained. Mortality was measured and determined after injecting A. hydrophila containing each of eight different profiles to the common carp fry. The mortality rate of infected fish with lipﶨ爇窰 and lip-/ahyBﶧꬃ profiles was higher from other groups and control group. In diseased fish, signs of darkening skin, shortening of gills and abdominal and vent petechial hemorrhages were observed. These results indicate that aerolysin, elastase and lipase virulence genes increase the pathogenicity of the A. hydrophila.