Siderophores Produced by the Fish Pathogen Flavobacterium columnare Strain MS-FC-4 Are Not Essential for Its Virulence.

Flavobacterium columnare causes columnaris disease in wild and aquaculture-reared freshwater fish. F. columnare virulence mechanisms are not well understood, and current methods to control columnaris disease are inadequate. Iron acquisition from the host is important for the pathogenicity and virulence of many bacterial pathogens. F. columnare iron acquisition has not been studied in detail. We identified genes predicted to function in siderophore production for ferric iron uptake. Genes predicted to encode the proteins needed for siderophore synthesis, export, uptake, and regulation were deleted from F. columnare strain MS-FC-4. The mutants were examined for defects in siderophore production, for growth defects in iron-limited conditions, and for virulence against zebrafish and rainbow trout. Mutants lacking all siderophore activity were obtained. These mutants displayed growth defects when cultured under iron-limited conditions, but they retained virulence against zebrafish and rainbow trout similar to that exhibited by the wild type, indicating that the F. columnare MS-FC-4 siderophores are not required for virulence under the conditions tested. IMPORTANCE Columnaris disease, which is caused by Flavobacterium columnare, is a major problem for freshwater aquaculture. Little is known regarding F. columnare virulence factors, and control measures are limited. Iron acquisition mechanisms such as siderophores are important for virulence of other pathogens. We identified F. columnare siderophore biosynthesis, export, and uptake genes. Deletion of these genes eliminated siderophore production and resulted in growth defects under iron-limited conditions but did not alter virulence in rainbow trout or zebrafish. The results indicate that the F. columnare strain MS-FC-4 siderophores are not critical virulence factors under the conditions tested but may be important for survival under iron-limited conditions in natural aquatic environments or aquaculture systems.