Soon after the introduction of methicillin for the treatment of infections caused by penicillinase-producing staphylococci, methicillin-resistant Staphylococcus aureus (MRSA) were isolated. MRSA have acquired the mecA gene which confers clinical resistance to all β-lactam antibiotics. Since their emergence MRSA have acquired additional resistance determinants by mutation, transposition and plasmid transfer. Until recently the only antibiotic to which MRSA had not acquired resistance was vancomycin. However, recent reports of low-level vancomycin-resistant MRSA may presage limitations in the effectiveness of vancomycin in the treatment of staphylococcal infections. While S. aureus is a well adapted pathogen, some strains of MRSA appear to possess an enhanced ability to spread within hospitals; these strains are sometimes referred to as ‘epidemic strains’. The genetic characteristics which confer epidemicity on these strains have not yet been identified.
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W.B. Grubb (1998) studied this question.