Isolation and identification of pathogenic Aeromonas in Clarias fuscus and analysis of the correlation between its pathogenicity and virulence genotypes

This study investigated the pathogenic bacteria of Clarias fuscus with fulminant death in Nanning City, Pubei County and Yulin City of Guangxi, and analysed the effect of the six virulence genes on its pathogenicity. Pathogen was isolated from the heart, liver and some other parts of the diseased fish by using conventional method, then an artificial infection test was measured on the pathogenicity of the isolated strains. Identification of the isolates was confirmed by API 20NE analysis and 16S rRNA gene sequencing, including a PCR assay of detecting six kinds of virulence genes of the isolates. Results showed that five pathogenic strains were isolated from the diseased fish, including three strains of Aeromonas hydrophila and two strains of Aeromonas sobria. The three strains of Aeromonas hydrophila shared the highest similarity (99.8%) with Aeromonas hydrophila standard strain ATCC 7966 (CP000462). Meanwhile the two strains of Aeromonas sobria shared the highest similarity (99.9%) with Aeromonas sobria standard strain NO.106 (AB472903.1). Among the five strains, detection rates of six virulence genes were as follows. Hundred percent of the strains carried gene Act and Aer. Eighty percent carried gene ahal, hly and Alt. Only twenty percent of the strains carried gene ahp. The five Aeromonas strains contained three virulence genotypes in all, including the main genotype Act⁺ahal⁺hly⁺Alt⁺ahp^-Aer⁺ of three strains (accounting for 60% of all the strains), as well as the genotype Act⁺ahal⁺hly⁺Alt⁺ahp⁺Aer⁺ and Act⁺ahal^-hly^-Alt^-ahp^-Aer⁺ of one strain (each accounting for 20% of all the strains). Strain which carried all the six detected virulence genes had the highest pathogenicity, whereas the one carrying only Act and Aer had the minimal pathogenicity. Gene ahp played an important role in pathogenicity of strains, and the pathogenicity of pathogenic bacteria resulted from the synergistic reaction of multiple virulence genes.