In recent decades, the notorious pathogen Staphylococcus aureus has become progressively more contagious, more virulent, and more resistant to antibiotics. This implies a rather dynamic evolutionary capability, representing a remarkable level of genomic plasticity, most probably maintained by horizontal gene transfer. Here we report that the staphylococcal pathogenicity islands have a dual role in gene transfer: they not only mediate their own transfer, but they can independently direct the transfer of unlinked chromosomal segments containing virulence genes. While transfer of the island itself requires specific helper phages, transfer of unlinked chromosomal segments does not, so potentially any pac-type phage will serve. These results reveal that SaPIs can increase the horizontal exchange of accessory genes associated with disease and may shape pathogen genomes beyond the confines of their attachment sites.
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