ABSTRACT
Methicillin-resistant Staphylococcus aureus (MRSA) is defined by minimum inhibitory concentrations (MIC) of methicillin of 16 f..Lg/ml or more or oxacillin 4 f..Lg/ml or more (1). Strains of MRSA possess the mecA gene (1,2). This chromosomal gene encodes an altered enzyme, termed penicillin-binding protein 2a (or PBP 2'), which has a low affinity for all beta-lactam antibiotics. This feature, presumably, allows the enzyme to perform essential functions in construction of the gram-positive cell wall, even in the presence of methicillin and other beta-lactam antibiotics. As a rule, strains of MRSA also possess resistance determinants for many other antimicrobial agents; in fact, until recently, only vancomycin provided reliable therapy for serious infections caused by this organism. This extraordinary level of resistance combined with the inherent virulence of S. aureus accounts for the level of interest and concern generated by MRSA.
