Flavobacterium columnare Ferric Iron Uptake Systems are Required for Virulence

Authors: CONRAD, RACHEL - University Of Wisconsin; Evenhuis, Jason; Lipscomb, Ryan; PEREZ-PASCUAL, DAVID - Institut Pasteur - France; STEVICK, REBECCA - University Of Wisconsin; Birkett, Clayton; GHIGO, JEAN-MARC - Institut Pasteur - France; MCBRIDE, MARK - University Of Wisconsin

Submitted to: Frontiers in Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/17/2022
Publication Date: 10/17/2022
Citation: Conrad, R.A., Evenhuis, J., Lipscomb, R.S., Perez-Pascual, D., Stevick, R.J., Birkett, C., Ghigo, J., Mcbride, M.J. 2022. Flavobacterium columnare ferric iron uptake systems are required for virulence. Frontiers in Microbiology. Volume 12. https://doi.org/10.3389/fcimb.2022.1029833.
DOI: https://doi.org/10.3389/fcimb.2022.1029833

Interpretive Summary: Acquisition and utilization of iron is important for proper Flavobacterium columnare function. Several genes were identified, through sequencing, that function in the acquisition and uptake of heme, a binding molecule of iron from blood, in F. columnare. Several of these genes were deleted individually or in combination and tested for changes in growth and virulence. Mutants with individual mutations exhibited some growth defects but no observed change in virulence against zebrafish or rainbow trout. Mutants with multiple gene deletions did lose virulence as compared to the non-mutated parent. This study may inform the development of safe and protective vaccines against F. columnare in aquaculture.

Technical Abstract: Flavobacterium columnare, which causes columnaris disease, is one of the costliest pathogens in the freshwater fish-farming industry. The virulence mechanisms of F. columnare are not well understood and current methods to control columnaris outbreaks are inadequate. Iron is an essential nutrient needed for metabolic processes and is often required for bacterial virulence. F. columnare produces siderophores that bind ferric iron for transport into the cell. The genes needed for siderophore production have been identified, but other components involved in F. columnare iron uptake have not been studied in detail. We identified the genes encoding the predicted secreted heme-binding protein HmuY, the outer membrane iron receptors FhuA, FhuE, and FecA, and components of an ATP binding cassette (ABC) transporter predicted to transport ferric iron across the cytoplasmic membrane. Deletion mutants were constructed and examined for growth defects under iron-limited conditions and for virulence against zebrafish and rainbow trout. Mutants with deletions in genes encoding outer membrane receptors, and ABC transporter components exhibited growth defects under iron-limited conditions. Mutants lacking multiple outer membrane receptors, the ABC transporter, or HmuY retained virulence against zebrafish and rainbow trout similar to that exhibited by the wild type. Some mutants predicted to be deficient in multiple steps of iron uptake exhibited decreased virulence. Survivors of exposure to such mutants were partially protected against later infection by wild-type F. columnare.