Effect of source of Mueller-Hinton agar on detection of oxacillin resistance in Staphylococcus aureus using a screening methodology

Dehydrated Mueller-Hinton agar from five different manufacturers was used to prepare plates containing 4% NaCl and 6 micrograms of oxacillin per ml to screen for oxacillin resistance in Staphylococcus aureus. A total of 137 isolates which included 109 oxacillin-resistant isolates obtained from at least eight geographic areas were examined. Results were compared to MICs obtained by using a standard broth microdilution method. Results obtained with screening plates prepared with dehydrated media from three of the manufacturers showed 100% correlation with MICs in detecting oxacillin-resistant S. aureus, and plates prepared with the remaining two media identified oxacillin resistance in 90 and 99% of the resistant isolates, respectively. None of the oxacillin-susceptible isolates grew on any of the screening plates. This study demonstrated that oxacillin screening plates are reliable for detecting oxacillin-resistant S. aureus; however, the source of Mueller-Hinton agar can influence the results.