Virulence properties and pathogenicity of Flavobacterium columnare in hybrid red tilapia (Oreochromis sp.).

Flavobacterium columnare is the causative organism of columnaris disease in freshwater fish, which may exhibit differences in pathogenicity. The virulence properties and the pathogenicity of F. columnare in juvenile hybrid red tilapia were investigated using a collection of F. columnare isolates recovered from diseased fish. Ten isolates were selected to perform immersion challenge assay in juvenile tilapia. The pathogenicity was highly diverse. Subsequently, three bacterial isolates exhibited different pathogenicity levels, consisting of high, moderate and low virulence (100%, 80% and 0% mortality respectively) and were subjected to virulence properties evaluation e.g. the median lethal dose (LD50), bacterial adhesion, capsular production and biofilm formation. Interestingly, the adhesion ability to the gill surface, biofilm formation and the production of capsular polysaccharide are significantly associated with the highly pathogenic traits of F. columnare. These findings suggest that efficient columnaris disease control in farmed fish should target these pivotal virulence properties.