The reliability of methicillin sensitivity tests on four culture media

The influence of culture medium, incubation temperature, and antibiotic content on the reliability of methicillin disc-sensitivity tests on strains of Staphylococcus aureus was evaluated. Tests were performed with an inoculum giving semiconfluent growth of colonies on four media, with 5-mug and 10-mug methicillin discs at incubation temperatures of 30, 35, and 37 degrees C. With Sensitest agar and Mueller-Hinton agar at 35 degrees C or below, methicillin resistance was reliably detected. On DST agar and Wellcotest agar discrimination between sensitive and resistant strains was possible only at 30 degrees C. On Wellcotest agar and DST agar it was not possible to detect methicillin resistance in one resistant strain at any temperature. For sensitive and resistant strains, the use of a 5-mug disc allowed greater differentiation than the 10-mug disc.