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) an d high-inoculum (10(9) CFU) disk diffusion tests. By the standard inoc ulum disk diffusion test, 10 of 67 (15%) isolates of oxacillin-resista nt Staphylococcus aureus and 3 of 47 (6%) isolates of Staphylococcus e pidermidis were falsely susceptible after 24 h of incubation at 35 deg rees C. By the high-inoculum disk diffusion test (10(9) CFU), 4 of the 10 isolates of S. aureus remained falsely susceptible, whereas none o f the isolates of S. epidermidis was falsely susceptible. Of the 10 is olates of S. aureus falsely susceptible by the standard disk test, onl y one remained falsely susceptible after an additional 24 h of incubat ion 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 inc ubation at 35 degrees C became resistant after an additional 24 h of i ncubation at 22 degrees C. Thus, extended incubation of both the stand ard and high-inoculum disk diffusion tests increased their accuracy in detecting oxacillin resistance. All isolates of oxacillin-resistant s taphylococci were accurately detected with the Vitek software upgrades (6.1 and 7.1) of the GPS-SA card.