MICROBIOLOGY AND MOLECULAR FEATURES OF METHICILLIN RESISTANT Staphylococcus aureus Methicillin resistant strains of Staphylococci were identified immediately upon the introduction of methicillin into clinical practice. The first outbreaks of infection ocurred in European hospitals in the early 1960. A chromosomal DNA "mec" gen is present in methicillin resistant strains. This mec contains mecA, the structural gene for penicilling binding protein 2a (PBP 2a) and mecI, mec R1 regulatory elements controlling mecA transcription. A distinctive feature of methicillin resistance is its heterogeneous nature, numerous genes influence in this phenotype: fem factors (A, B, C, D, E and F). The methods for detection of methicillin resistance rely on modified culture conditions to enhance the expression of resistance, use of oxacillin disk, and incubation at 30 or 35°C for 24 hours instead of 16 to 18 hours.

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