Optimal inoculation methods and quality control for the NCCLS oxacillin agar screen test for detection of oxacillin resistance in Staphylococcus aureus

To define more precisely the inoculation methods to be used in the oxacilli n screen test for Staphylococcus aureus, we tested agar screen plates prepa red in house with 6 mug of oxacillin/ml and 4% NaCl using the four differen t inoculation methods that would most likely be used by clinical laboratori es. The organisms selected for testing were 19 heteroresistant mecA-produci ng strains and 41 non-mecA-producing strains for which oxacillin MICs were near the susceptible breakpoint. The inoculation method that was preferred by all four readers and that resulted in the best combination of sensitivit y and specificity was a 1-mul loopful of a 0.5 McFarland suspension. A seco nd objective of the study was to then use this method to inoculate plates f rom five different manufacturers of commercially prepared media. Although a ll commercial media performed with acceptable sensitivity compared to the r eference lot, one of the commercial lots demonstrated a lack of specificity . Those lots of oxacillin screen medium that fail to grow heteroresistant s trains can be detected by using S. aureus ATCC 43300 as a positive control in the test and by using transmitted light to carefully examine the plates for any growth. However, lack of specificity with commercial lots may be di fficult to detect using any of the current quality control organisms.