Abstract:
The causative agent of columnarsis disease, Flavobacterium columnare is a bacterial pathogen affecting many wild and farmed freshwater fishes, and subsequently resulting in significant economic losses globally. To date, various studies have been elucidated the pathogenesis of columnaris disease. However, comprehension of virulence factors and pathogenesis of F. columnare are still unclear. The objective of this study was to investigate pathogenicity of F. columnare isolates recovered from diseased red tilapia and some of their virulence-associated properties. Six specific primer pairs targeting each particular putative virulence gene were designed. The PCR results showed two distinct putative virulence gene profiles among 51 F. columnare isolates. However, it seems to be unrelated with pathogenicity in red tilapia. The LD50 of high, moderate and low virulence isolates were 6.10 log10CFU, 6.65 log10CFU and more than 8 log10CFU ml-1 of rearing water, respectively. The biofilm formation and adherent ability were significantly correlated with the virulence of F. columnare. The protease activity of F. columnare on skim milk agar was strong, however, lack of association with the levels of pathogenicity. Besides, high and moderate virulent isolates showed the high thickness of polysaccharide capsule when compare with low virulence isolate. In conclusion, adherences and thickness of polysaccharide capsule might play an important role in F. columnare virulence and infection in red tilapia.
