MRSA = methicillin resistance S. aureus
presence of mec gene --> encodes the penicillin binding protein 2a (PBP2a), a peptidoglycan transpeptidase --> leads to resistance to nafcillin, oxacillin, and most cephalosporins
clinical micro labs define methicillin resistance as an oxacillin MIC > (or equal to) 4 mcg/mL
methicillin is no longer in use!
MSSA = methicillin susceptible S. aureus
no mec gene --> susceptible to nafcillin, oxacillin, and most cephalosporins
about 50 % of all S. aureus are methicillin-resistant!!