Methicillin-resistant staphylococci: detection methods and treatment of infections

Soon after methicillin became available, resistance was reported in Staphylococcus epidermidis and Staphylococcus aureus . Resistance was not the result of destruction of the antibiotic by beta -lactamase and was termed intrinsic. Most strains of methicillin-resistant staphylococci are heterogeneous in their phenotypic expression of the resistance despite genetic homogeneity. Typically, 1 cell in a population of 10 super(4) to 10 super(8) expresses resistance as defined by its ability to grow in the presence of high concentrations of beta-lactam antibiotic. Heterogeneous strains may appear susceptible to beta-lactam antibiotics under standard susceptibility testing conditions. Expression of resistance is enhanced by passage in beta-lactam antibiotics because the susceptible subpopulation is eliminated and the highly resistant subpopulation is selected out. These antibiotic-selected cells are more uniformly resistant than the parent strain, but the trait is unstable.