One method for detecting methicillin (oxacillin)-resistant Staphylococcus aureus in clinical laboratories is using oxacillin salt agar. This method detects mecA-mediated resistance using oxacillin salt agar for S. aureus only (CLSI 2024).
The image illustrates a Mueller-Hinton agar plate containing 6 μg/mL of oxacillin and 4% NaCl, previously inoculated with a strain of Staphylococcus aureus. Oxacillin is a marker for methicillin resistance because it is more stable in the agar medium. Growth on this screening medium is presumptive for methicillin (oxacillin) resistance. Thus, in the presence of growth, as shown here, a follow-up minimum inhibitory concentration (MIC) test must be performed to determine the exact resistance level.
