Oxacillin disk testing is no longer reliable for S. aureus and S. lugdunensis (CLSI 2024). If testing is required, the MIC susceptibility test method must be employed. Breakpoints for MICs are different for these two organisms compared with other staphylococci.
The disk diffusion test can detect methicillin (oxacillin)-resistant Staphylococcus (not aureus or lugdunensis). Illustrated in the image is the surface of a Mueller-Hinton agar plate previously inoculated with a strain of S. epidermidis suspected of being methicillin (oxacillin)-resistant. Although the zone of inhibition is at the borderline for resistance per CLSI (18 mm), small colonies within the zone of inhibition (yellow arrows) indicate the presence of heteroresistant strains. Therefore, the interpretation is "methicillin (oxacillin)-resistant" staphylococci (reported as oxacillin-resistant), even though the zone diameter appears to be adequate. Detecting the heteroresistant strains requires minimum inhibitory concentration (MIC) studies.
