Comparison of several methods to determine methicillin-resistance in Staphylococcus aureus with focus on borderline strains.

We compared the performance of several phenotypic tests to detect methicillin-resistant Staphylococcus aureus, with special focus on borderline strains. The reliability of the agar screen oxacillin and BBL Crystal tests was asserted for all methicillin-susceptible (n = 25), -resistant (n = 29) and borderline beta-lactamase-hyperproducer (n = 10) strains. Whereas these tests failed to detect 4 of 5 rare borderline strains containing few cells with high-level methicillin resistance (i.e., a frequency of 10(-7)-10(-8)), a "two-temperature" disk diffusion method, performed simultaneously at 35 and 42 degrees C, detected all of such strains.