Shortly after the introduction of methicillin in 1959, isolates resistant to this agent were reported. Outbreaks of methicillin-resistant Staphylococcus aureus (MRSA) infections occurred in Europe in the early 1960s. Since these original descriptions, MRSA as well as coagulase-negative staphylococci, which are commonly resistant to methicillin, have emerged as major nosocomial and, in the case of MRSA, community-acquired pathogens.
Methicillin resistance is defined in the clinical microbiology laboratory as an oxacillin minimum inhibitory concentration ≥4 mcg/mL. Isolates resistant to oxacillin or methicillin are also resistant to all beta-lactam agents, including cephalosporins.
Methicillin resistance requires the presence of the mec gene; strains lacking a mec gene are not methicillin resistant. The structural component of the mec gene, mecA, encodes the penicillin-binding protein 2a (PBP2a) that establishes resistance to methicillin and other semisynthetic penicillinase resistant beta-lactams.
The most accurate methods to detect MRSA are polymerase chain reaction for detection of the mecA gene and latex agglutination tests for the protein product of mecA, PBP2a. When these tests are not available, traditional microbiology laboratory techniques are acceptable, such as oxacillin-salt agar screening plates and cefoxitin disk diffusion tests.