All methicillin-resistant Staphylococcus aureus (MRSA) are not equal

Abstract

New typing techniques are giving a greater understanding of the evolution and epidemiology of methicillin-resistant Staphylococcus aureus (MRSA). A comparison of the overall genetic background of isolates is now possible using MultiLocus sequence typing (MLST). However, even isolates with the same genetic background can be different from each other because of the acquisition of antibiotic-resistance and pathogenicity determinants. Sequencing of the region involved with methicillin resistance, the mec region or staphylococcal cassette chromosome mec (SCCmec), has demonstrated that there are different SCCmecs. It has been found that strains with similar genetic backgrounds can have different SCCmecs and that strains with different genetic background can have similar SCCmecs. This, and molecular evidence, indicates that SCCmecs are mobile elements that are able to move between cells by an as yet unidentified mechanism. Typing has also demonstrated that some strains, known as epidemic MRSA (EMRSA), spread more readily in hospitals and health care institutions. However, it is not known why they have this ability. Whereas in the past MRSAs were regarded as being associated with health care institutions, strains have emerged in the community that are quite different from those associated with health care institutions. These strains are not multiply resistant to antimicrobials like most of the strains associated with health care institutions; however, there is concern that they will acquire additional resistance determinants and present problems for the treatment of staphylococcal infections. Some EMRSA are not multiply resistant and, on their resistance profiles, could be mistaken for community MRSA. This highlights the importance of typing in identifying and tracing MRSA for infection control.