Sequence analysis revealed a complex picture, as the various CNS species were found to differ as widely from each other as any differed from S. aureus . MecA has no allelic counterpart in McS isolates and is considered to have entered medically important staphylococci from other species or genera. MecA occurs within a larger segment of DNA also lacking counterpart in McS cells, the mec region. We have found that MRSA fall into several fundamental groups with regard to mec region structure, resulting from at least five instances in evolutionary time of transfer of large blocks of mec region DNA between CNS species and S.aureus. Superimposed upon this oligoclonal origin have been intra-species evolution of the mec region.
Staphylococcal infections occur variably in outbreak or sporadic form.
For purposes of hospital infection control, as well as monitoring of the community, it is important to be able to track lineages. We have evaluated sequence analysis of a polymorphic segment of the mec region as a way to track both SA and CNS lineages. An emergent infectious disease problem concerns a newly described subspecies of the CNS S. hominis , S. hominis novobiosepticus . Its designation arises from its properties of being resistant to novobiocin and its unusually high virulence. The precise relationship between the sequevar and subspecies classifications, and the nature of the increased virulence of S. hominis novobiosepticus , are complex and under study.