Abstract
Horizontal gene transfer is a key step in the evolution of bacterial pathogens. Besides phages and plasmids, pathogenicity islands (PAIs) are subjected to horizontal transfer. The transfer mechanisms of PAIs within a certain bacterial species or between different species are still not well understood. This study is focused on the High-Pathogenicity Island (HPI), which is a PAI widely spread among extraintestinal pathogenic Escherichia coli and serves as a model for horizontal transfer of PAIs in general. We applied a phylogenetic approach using multilocus sequence typing on HPI-positive and -negative natural E. coli isolates representative of the species diversity to infer the mechanism of horizontal HPI transfer within the E. coli species. In each strain, the partial nucleotide sequences of 6 HPI–encoded genes and 6 housekeeping genes of the genomic backbone, as well as DNA fragments immediately upstream and downstream of the HPI were compared. This revealed that the HPI is not solely vertically transmitted, but that recombination of large DNA fragments beyond the HPI plays a major role in the spread of the HPI within E. coli species. In support of the results of the phylogenetic analyses, we experimentally demonstrated that HPI can be transferred between different E. coli strains by F-plasmid mediated mobilization. Sequencing of the chromosomal DNA regions immediately upstream and downstream of the HPI in the recipient strain indicated that the HPI was transferred and integrated together with HPI–flanking DNA regions of the donor strain. The results of this study demonstrate for the first time that conjugative transfer and homologous DNA recombination play a major role in horizontal transfer of a pathogenicity island within the species E. coli.
Highlights
The diversity of bacteria is caused by various genetic mechanisms including point mutations, genetic rearrangements and horizontal gene transfer (HGT), all of which represent driving forces of bacterial evolution [1]
This is consistent with the distribution of the High-Pathogenicity Island (HPI) to extraintestinal pathogenic E. coli (ExPEC), as phylogenetic groups B2 and D predominantly consist of ExPEC strains causing urinary tract infections, septicaemia and meningitis
In all except one HPI-positive E. coli reference (ECOR) strain (ECOR31), the HPI was found to be inserted at the asnT tRNA gene and to carry a unique deletion of the right border of the HPI leading to a loss of the direct repeat
Summary
The diversity of bacteria is caused by various genetic mechanisms including point mutations, genetic rearrangements and horizontal gene transfer (HGT), all of which represent driving forces of bacterial evolution [1]. HGT can effectively change the life style of bacterial species This is true for bacterial pathogens, where virulence is linked to acquisition of virulence determinants by horizontal transfer and the evolution of bacterial virulence may be viewed as a process of adaptation that enables a pathogenic lifestyle. More widespread means of HGT include the transfer of large DNA elements such as plasmids, phages and genomic islands (e.g. pathogenicity islands, PAIs) [3,4]. These PAIs are especially important in processes leading to new bacterial pathotypes as the incorporation of a PAI can, in a single step, transform a normally benign organism into a pathogen [5]
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