Evaluation of differential inoculum disk diffusion method and Vitek GPS-SA card for detection of oxacillin-resistant staphylococci

This study was conducted in order to compare the accuracy of detection of oxacillin-resistant staphylococci, defined by microdilution MICs, population analyses, and mec gene hybridization, with the Vitek GPS-SA Susceptibility Card with that of the standard inoculum (10(7) CFU) and high-inoculum (10(9) CFU) disk diffusion tests. By the standard inoculum disk diffusion test, 10 of 67 (15%) isolates of oxacillin-resistant Staphylococcus aureus and 3 of 47 (6%) isolates of Staphylococcus epidermidis were falsely susceptible after 24 h of incubation at 35 degrees C. By the high-inoculum disk diffusion test (10(9) CFU), 4 of the 10 isolates of S. aureus remained falsely susceptible, whereas none of the isolates of S. epidermidis was falsely susceptible. Of the 10 isolates of S. aureus falsely susceptible by the standard disk test, only one remained falsely susceptible after an additional 24 h of incubation at 22 degrees C. All four isolates of S. aureus that were falsely susceptible by the high-inoculum disk diffusion test after 24 h of incubation at 35 degrees C became resistant after an additional 24 h of incubation at 22 degrees C. Thus, extended incubation of both the standard and high-inoculum disk diffusion tests increased their accuracy in detecting oxacillin resistance. All isolates of oxacillin-resistant staphylococci were accurately detected with the Vitek software upgrades (6.1 and 7.1) of the GPS-SA card.