[A simple method for the determination of the class of expression of methicillin resistance]

Four stable classes of phenotypic expression in methicillin resistant Staphylococcus aureus were described: three of them showed heterogeneous resistance. The fourth was homogeneous. Methicillin resistance depend on the gene mecA encoding an altered penicillin-binding protein. The phenotypic expression was controlled by chromosomic factors (fem and aux genes) and was modified by test conditions: (temperature - pH - NaCl 4%). The conventional method for determination of these different classes was the population analysis method using methicillin as antibiotic indicator. This fastidious method cannot use in routine laboratory. In this work, the authors developed a common method using the diffusion agar method. Four antibiotic disks (methicillin 2.5, 15, 100 micrograms oxacillin 30 micrograms) and three incubation temperatures (30 - 35 - 40 degrees C). The determination of the phenotypic classes could be an epidemiologic marker for the survey of epidemic MRSA in hospital.