Genetic and virulence diversity of Flavobacterium columnare
Flavobacterium columnare is a freshwater fish bacterium responsible for columnaris disease, the second leading cause of mortality in pond raised catfish in the southeastern United States. Pulsed-field gel electrophoresis (PFGE) is a particularly powerful tool in epidemiology and is now regarded as the gold standard for molecular typing of microorganisms. We developed methods for conducting PFGE on F. columnare, and determined its efficacy for characterizing F. columnare strains isolated from different locations in the Southeastern United States. Virulence diversity was observed in two different immersion challenge experiments conducted with 16 different isolates in channel catfish fingerlings. A direct correlation was found between the PFGE clustered groups and virulence. In summary, our results suggest that two genetic divisions of F. columnare channel catfish isolates exist, one that contains strains that are “primary” pathogens of channel catfish (Group A), and another that are “secondary” or opportunistic pathogens of catfish (Group B).