Detection of intrinsically resistant (heteroresistant) Staphylococcus aureus with the Sceptor and AutoMicrobic systems.

Modified procedures for the Sceptor Gram-Positive MIC Panel and the Vitek AutoMicrobic System GPS-M Card were evaluated for their ability to detect methicillin-resistant (heteroresistant) Staphylococcus aureus. A total of 398 clinical isolates (including 222 methicillin-resistant S. aureus) obtained from 10 hospitals were tested. Both systems had 2% NaCl in the oxacillin wells. Sceptor MIC panels were inoculated with an organism suspension prepared from an 18- to 24-h blood agar plate and were inoculated for a full 24 h at 35 degrees C before MICs were read. All methicillin-resistant S. aureus isolates were detected as resistant to oxacillin at greater than or equal to 8 micrograms/ml by the Sceptor method and at greater than 2 micrograms/ml by the Vitek method. All 176 oxacillin-susceptible, methicillin-susceptible S. aureus isolates were correctly distinguished from methicillin-resistant S. aureus isolates by Sceptor. However, with the Vitek system 29 methicillin-susceptible S. aureus isolates tested as falsely resistant to oxacillin and four isolates tested as falsely resistant to vancomycin. The modified testing procedure with the Sceptor system can be used reliably for accurate susceptibility testing of methicillin-resistant and methicillin-susceptible S. aureus. The Vitek GPS-M card does not accurately discriminate between methicillin-resistant and methicillin-susceptible S. aureus with an oxacillin breakpoint of greater than 2 micrograms/ml.