DISCRIMINATION OF METHICILLIN-RESISTANT STAPHYLOCOCCUS-AUREUS FROM BORDERLINE-RESISTANT AND SUSCEPTIBLE ISOLATES BY DIFFERENT METHODS

Methicillin-resistant Staphylococcus aureus (MRSA) are important nosoc omial pathogens, Diseases caused by these resistant bacteria frequentl y are serious and there is a need to control the spread of epidemic MR SA clones in hospitals. However, detection is complicated by the fact that expression of the resistance is variable and, commonly, heterogen eous within strains. The reliability of several tests recommended to d iscriminate heterogeneous MRSA isolates from borderline-resistant and susceptible strains was evaluated. Screening for growth on agar with m ethicillin 25 mg/L was the only method that detected all MRSA strains tested, but screening on agar with methicillin 10 mg/L or oxacillin 6 mg/L detected all but one of 10 heterogeneously resistant strains test ed. None of the borderline-resistant nor any truly susceptible staphyl ococci tested grew on any of these screening plates.