Evaluation of MicroScan MIC panels for detection of oxacillin-resistant staphylococci

Clinical isolates of staphylococci (420 Staphylococcus aureus isolates and 248 coagulase-negative staphylococci) were tested by both MicroScan MIC panels (MicroScan, West Sacramento, Calif.) and an oxacillin agar screen (Mueller-Hinton agar [Difco Laboratories, Detroit, Mich.] containing 6 micrograms of oxacillin per ml and 4% NaCl) to evaluate the ability of MicroScan to detect oxacillin-resistant strains. MicroScan panels and oxacillin agar screen plates were incubated at 35 degrees C for 24 h and at 30 degrees C for an additional 24 h. Endpoints were recorded at 24 and 48 h. By MicroScan, 23 (5.5%) and 30 (7%) S. aureus isolates and 161 (65%) and 162 (65%) coagulase-negative staphylococci were oxacillin resistant at 24 and 48 h, respectively. At both 24 and 48 h, 23 (5.5%) S. aureus isolates and 162 (65%) coagulase-negative staphylococci were resistant by the oxacillin agar screen. Five strains for which the oxacillin MIC was 2 or 4 micrograms/ml and eight strains resistant to oxacillin only at 48 h were further evaluated by broth macrodilution testing for oxacillin with and without clavulanic acid, by oxacillin and amoxicillin-clavulanic acid disk diffusion, and by oxacillin agar screen comparing Mueller-Hinton agars purchased from Difco and BBL Microbiology Systems, Cockeysville, Md. By this additional testing, all 10 S. aureus isolates and 1 of 3 coagulase-negative staphylococci examined produced increased amounts of beta-lactamase. One coagulase-negative staphylococcus appeared to be truly intermediately oxacillin susceptible. There was no significant difference in the rate of detection of oxacillin resistance between MicroScan and the agar screen. MicroScan panels should be incubated for 24 h only, because prolonged incubation caused strains producing excessive amounts of beta-lactamase to appear to be falsely oxacillin resistant.