Bordetella pertussisTonB, a Bvg-Independent Virulence Determinant

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Abstract
In gram-negative bacteria, high-affinity iron uptake requires the TonB/ExbB/ExbD envelope complex to release iron chelates from their specific outer membrane receptors into the periplasm. Based on sequence similarities, theBordetella pertussis tonB exbB exbDlocus was identified on a cloned DNA fragment. The tight organization of the three genes suggests that they are cotranscribed. A putative Fur-binding sequence located upstream fromtonBwas detected in a Fur titration assay, indicating that thetonB exbB exbDoperon may be Fur-repressed in high-iron growth conditions. Putative structural genes of the β-subunit of the histone-like protein HU and of a new two-component regulatory system were identified upstream fromtonBand downstream fromexbD, respectively. AB. pertussis ΔtonB exbB::Kmrmutant was constructed by allelic exchange and characterized. The mutant was impaired for growth in low-iron medium in vitro and could not use ferrichrome, desferal, or hemin as iron sources. Levels of production of the major bacterial toxins and adhesins were similar in the TonB+/TonB−pair. The ΔtonB exbBmutant was still responsive to chemical modulators of virulence; thus, the BvgA/BvgS two-component system is not TonB dependent. Nevertheless, in vivo in the mouse respiratory infection model, the colonization ability of the mutant was reduced compared to the parental strain.