Staphylococcal plasmids, transposable and integrative elements

As has been the case for other bacterial genera, studies of the genetic basis of staphylococcal pathogenicity and antimicrobial resistance highlighted the presence of determinants not uniformly represented in the genomes of all staphylococcal strains. Although not a fundamental requirement for survival per se, such accessory DNA usually encodes functions that are advantageous in a particular environmental niche. The extent of the accessory component of the genome has been revealed by comparative analysis of whole Staphylococcus aureus genome sequences, which indicate that it can constitute in excess of 20% of a strain’s genetic makeup (1-5). It is clear that the acquisition, maintenance, and dissemination of accessory functions have been central to the ongoing success of staphylococci as pathogens. These processes are underpinned by interactions between mobile genetic elements, such as insertion sequences and transposons that mediate intracellular movement of DNA, and plasmids, bacteriophage and integrative and conjugative elements (ICEs) that facilitate intercellular DNA mobility.