The carbohydrate L-rhamnose promotes biofilm formation which enhances Flavobacterium columnare virulence

Flavobacterium columnare, the causative agent of columnaris disease causes substantial mortality worldwide in numerous freshwater finfish species. Due to its global significance and impact on the aquaculture industry, continual efforts to better understand basic mechanisms that contribute to disease are urgently needed. The current work sought to evaluate the effect of L-rhamnose on the growth characteristics of F. columnare.  While we initially did not observe any key changes between the in vitro growth or in the colonization of gill tissue by F. columnare when rhamnose- treated; it soon became apparent that the difference lies in the ability to form more robust biofilms when under the influence of the carbohydrate, L-rhamnose. In vitro biofilm assays demonstrated enhanced biofilm formation in a dose dependent manner when using either L-rhamnose or D-galactose but not with D-glucose. Quantitative PCR analyses of gliding motility genes early after L-rhamnose stimulation revealed a pattern of upregulation in planktonic cells to facilitate increased movement and attachment to the host; then during biofilm formation their initial downregulation and later upregulation once again to facilitate the different stages of development. The overall mortality observed in the in vivo bacterial challenges indicates that biofilm formation has a direct association to virulence. Our results allow for some conclusions to be made about the formation of F. columnare biofilms and their likely mechanism of pathogenicity.