Understanding and Controlling Methicillin-ResistantStaphylococcus aureusInfections

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Abstract
EXCERPT During the past four decades, methicillin‐resistant Staphylococcus aureus (MRSA) has spread throughout the world and has become highly endemic in many geographic areas. Recent studies of methicillin‐sensitive S. aureus (MSSA) and MRSA isolates collected over many years and analyzed by multilocus sequence typing (MLST), staphylococcal cassette chromosome mec (SCCmec) typing, and use of a computerized algorithm based on related sequence types (BURST) have revealed that the evolution and spread of MRSA occurred because of the introduction of the mobile SCCmec element into several different clones of MSSA.1 Early MRSA strains appear to have developed from a MSSA strain (ST250‐MSSA) that was prevalent in European countries, including Denmark, in the 1950s.1,2 Of interest, this MSSA ancestor to early MRSA strains is no longer prevalent among disease‐associated isolates. More recent epidemic strains of MRSA (EMRSA‐2, ‐6, ‐7, ‐12, ‐13, and ‐14) that spread effectively in hospitals during the 1980s and 1990s are indistinguishable by MLST, suggesting that a relatively small number of MRSA clones have unique qualities that facilitate their transmission over wide geographic areas. For example, EMRSA clones ST8‐MRSA‐III and ST239‐MRSA‐III have been recovered from patients in Finland, France, Germany, the Netherlands, Sweden, the United Kingdom, and the United States.1 One can conclude from the above findings that the occurrence of epidemics or high levels of endemicity observed in a given geographic area can be explained, at least in part, by whether strains with epidemic potential are circulating in healthcare facilities.

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