Virulence characteristics of Aeromonas veronii biovars isolated from infected freshwater goldfish (Carassius auratus)

Motile aeromonads are significant pathogens of freshwater ornamental fish, responsible for diverse types of infections. Among motile aeromonads, Aeromonas veronii is an important fish pathogenic bacterial species consisting of two biovars, veronii and sobria. In the present study,the pathogenicity of Aeromonas veronii isolated from infected goldfish (Carassius auratus) was assessed. A total of 68 bacterial isolates from 123 infected samples were presumptively identified as genus Aeromonas based on six biochemical tests. These isolates were recognized at the species level as A. veronii bv.sobria (5) and A. veronii bv.veronii (6) strains, and validated using the Moeller decarboxylase and dihydrolase test. In-vivo pathogenicity of A. veronii to C. auratus resulted in 100% morality in challenged host within one week of post experimental injection for both biovars. The isolates showed significant phenotypical activities such as the haemolytic, proteolytic, caseinase, nuclease, crystal violet binding activities and biofilm formation which are indicative of their virulence potential. Virulence genes such as the enterotoxin genes (act, alt and ast), haemolytic toxin genes (hlyA and aerA) and the gene of type IV pilus (tapA) were detected in both the biovars of A. veronii species, but ast and tapA genes were absent in A. veronii bv.sobria and A.veronii bv.veronii, respectively. All the isolates were resistant to penicillin-G, ampicillin, ticarcillin, amoxyclav, carbencillin, carbapenem, tetracycline, steroids, fluroquinolone, and trimethoprim. Although both the biovars were virulent, in- vivo study showed that A. veronii by. sobria was more virulent than A. veronii by. veronii in goldfish. Statement of relevance: Aeromonas veronii can cause significant mortality in freshwater ornamental fish resulting in economic losses. Understanding the processes of A. veronii pathogenicity and virulence will assist in controlling this pathogen and the spread of disease.