Pathogenicity of Edwardsiella tarda to olive flounder, Paralichthys olivaceus (Temminck & Schlegel)

The LD50 and cytotoxic and enzymatic activities of both cells and extracellular products (ECPs) of eight Edwardsiella tarda strains were determined and their bacterial superoxide dismutase gene (sodB) and catalase gene (katB) were sequenced. Strains were also examined for their ability to resist the immune responses of olive flounder, Paralichthys olivaceus. LD50 values of strains (FSW910410, KE1, 2, 3, 4, 5 and 6) in olive flounder ranged between 10(2.5) and 10(5.3) cfu (colony forming units) per fish. Unlike the avirulent strain SU100 (LD50 >= 10(7)), all pathogenic strains were able to survive in flounder serum and head kidney leucocytes (except for KE2). The virulent strains possessed type I sodB and katB, whereas SU100 had type II sodB but not katB. However, there was no difference between avirulent and virulent strains in haemolytic and cytotoxic activities. The results of this study demonstrated that the ability of E. tarda to resist complement activity and phagocytosis is conferred by its superoxide dismutase and catalase, which thus play an essential role in the pathogenicity of this bacterium. In addition genotyping of sodB and kat B proved to be a very useful tool to distinguish virulent from avirulent strains.