Evaluation of mannitol salt agar with oxacillin as a screening medium for methicillin-resistant Staphylococcus aureus

We evaluated the use of mannitol salt agar with oxacillin for use as a primary screening medium for the simultaneous detection and identification of methicillin-resistant Staphylococcus aureus in clinical surveillance specimens. Oxacillin agar dilution susceptibility tests with mannitol salt agar and Mueller-Hinton agar were performed in parallel with disk-agar diffusion testing on 95 oxacillin-susceptible and 105 oxacillin-resistant S. aureus stock isolates. MICs were found to be comparable, showing distinct separation of susceptible and resistant isolates into two groups with MICs of less than or equal to 2 and greater than or equal to 32 micrograms/ml, respectively. In accord with these findings, 4 micrograms of oxacillin per ml was selected for use in the screening medium. For performance evaluation, mannitol salt agar with 4 micrograms of oxacillin per ml was compared with mannitol salt agar without oxacillin by performing parallel screening tests on 153 clinical surveillance specimens. For detection of methicillin-resistant S. aureus, mannitol salt agar with 4 micrograms of oxacillin per ml was as sensitive as mannitol salt agar without oxacillin and required significantly fewer confirmatory tests. For primary identification of methicillin-resistant S. aureus, mannitol salt agar with 4 micrograms of oxacillin per ml was 6.4% false-positive and 1.1% false-negative, with a 93.6% positive predictive value. These findings indicate that mannitol salt agar with 4 micrograms of oxacillin per ml can be used as a reliable and cost-effective screening medium for the simultaneous detection and identification of methicillin-resistant S. aureus in clinical surveillance specimens.