Chondroitin AC lyase activity is related to virulence of fish pathogenic Flavobacterium columnare

The virulence of eight Flavobacterium columnare strains was studied to find correlations between several virulence‐related factors and virulence. Virulence was tested in vivo using rainbow trout, Oncorhynchus mykiss (Walbaum). Suggested virulence‐related factors such as production of the degradative enzyme chondroitin lyase, plasmid occurrence and adhesion capability were studied in vitro. Infection with the four most virulent strains resulted in 95–100% mortality within 114 h. Chondroitin lyase activity was found to be significantly related to the virulence of the strains at 25 °C and it was also shown to be temperature‐dependent, being higher at 25 °C than at 20 °C. Virulence was not plasmid associated. The adhesion capability of the strains in vitro varied substantially when tested on crude mucus‐coated slides and no statistical relationship between adhesion and virulence was found using this method.