Identification and functional characterization of two heme uptake systems required for virulence of the fish pathogen Flavobacterium psychrophilum

Abstract

Bacterial pathogens have a critical impact on aquaculture, a sector that accounts for half of the human fish consumption. Flavobacterium psychrophilum (phylum Bacteroidetes) is responsible for bacterial cold-water disease in salmonids worldwide. The molecular factors involved in host invasion, colonization and hemorrhagic septicaemia are mostly unknown. In this study, we identified two new TonB-dependent receptors, HfpR and BfpR, that are required for adaptation to iron conditions encountered during infection and for virulence in rainbow trout. Transcriptional analyses revealed that their expression is tightly controlled and upregulated under specific iron sources and concentrations. Characterization of deletion mutants showed that they act without redundancy: BfpR is required for optimal growth in the presence of high hemoglobin level, while HfpR confers the capacity to acquire nutrient iron from heme or hemoglobin under iron scarcity. The gene hfpY, co-transcribed with hfpR, encodes a protein related to the HmuY family. We demonstrated that HfpY binds heme and contributes significantly to host colonization and disease severity. Overall, these results are consistent with a model in which both BfpR and Hfp systems promote heme uptake and respond to distinct signals to adapt iron acquisition to the different stages of pathogenesis. Our findings give insight into the molecular basis of pathogenicity of a serious pathogen belonging to the understudied family Flavobacteriaceae and point to the newly identified heme receptors as promising targets for antibacterial development.