The high-pathogenicity island (HPI) promotes flagellum-mediated motility in extraintestinal pathogenic Escherichia coli

The key of success of extraintestinal pathogenic Escherichia coli (ExPEC) to colonize niches outside the intestinal tract and to establish infection is the coordinated action of numerous virulence and fitness factors. The so-called high-pathogenicity island (HPI), responsible for synthesis, secretion and uptake of the siderophore yersiniabactin, proved to be an important virulence determinant. In this study we investigated the interaction of the flagellum-mediated motility and the HPI. The impairment of yersiniabactin production by deletion of irp2 or ybtA affected significantly motility. The gain of yersiniabactin production improved motility in both pathogenic and non-pathogenic E. coli strains. The loss of flagella expression had no adverse effect on the HPI. Strikingly, external iron abundance was not able to suppress activation of the HPI during motility. The HPI activity of swarming bacteria was comparable to iron deplete conditions, and could even be maximized by supplementing excessive iron. This fact is the first description of a regulatory mechanism, which does not follow the known hierarchical regulation of siderophore systems. Transcriptional reporter fusions of the ybtA promoter demonstrated that the entire promoter region with all YbtA binding sites is necessary for complete induction in both HPI-positive and HPI-negative strains. Altogether, these results suggest that the HPI is part of a complex regulatory network, which orchestrates various virulence mechanisms to optimize the overall fitness of ExPEC.