Identification of the causative pathogen causing skin-ulcer in Silurus soldatovi and its pathogenicity

In order to investigate the cause and prevention measure of skin-ulcer of Silurus soldatovi in Heilongjiang River, the pathogenic bacteria were isolated from the liver, spleen and kidney of diseased S. soldatovi with conventional methods. Artificial infection experiment was carried out to prove the pathogenicity of isolated strains. The morphological, biochemical, molecular and characteristics, virulence genes and antimicrobial resistance to bacteria were examined. The results indicated that 3 pathogenic strains were isolated from diseased fish, named NY-8, NY-9 and NY-12, respectively. Artificial infection showed that NY-8 and NY-9 were highly pathogenic to experimental fish while NY-12 strain was less virulent to fish. Co-infection of 3 strains caused 100% mortality of fish with the same clinical signs as the natural case. Combining the biochemical and molecular characteristics of 3 strains, NY-8, NY-9 and NY-12 were determined to be Aeromonas veronii, A. salmonicida and A. hydrophila, respectively. The three strains contained two virulence genotypes, including hly+/aer+/act+/alt+/GCAT+ and hly+/aer-/act+/alt+/GCAT+. The pathogenicity of NY-8 and NY-9 carrying all five virulence genes was significantly higher than that of NY-12. Antimicrobial susceptibility tests showed that there were some differences in drug resistance spectrum in 3 strains. NY-8 and NY-9 were sensitive to 4 kinds of fluoroquinolones and resistant to aminoglycoside and nitrofurans antibiotics, while NY-12 was only sensitive to levofloxacin and florfenicol.