[Comparison of routine sensitivity tests and mecA gene analysis for determination of methicillin resistance in...

Currently, oxacillin disk diffusion test is the most frequently employed method to detect the methicillin resistance of Staphylococcus aureus isolates. However, due to some of the test conditions, errors may occur during the detection of heteroresistant bacteria. It is now widely accepted that lower incubation temperatures (< or = 35 degrees C) and media with a NaCl concentration of 2-4%, could facilitate detection. In our study, methicillin (oxacillin) susceptibilities of 125 S. aureus isolates were determined by the disk diffusion and microdilution tests as defined by National Committee for Clinical Laboratory Standards (NCCLS), and the results were compared with those of mecA gene analysis. In the routine susceptibility tests, 75 isolates were found to be methicillin resistant (MRSA), whereas 50 were found to be susceptible (MSSA). Various induction tests were performed to investigate the heterogeneous resistance among methicillin-susceptible isolates. These induction tests showed that the MIC values of seven isolates reached to the resistant levels, therefore these isolates should be accepted as "borderline oxacillin resistant S. aureus" isolates lacking the mecA gene. The susceptibility tests and mecA gene analysis of the remaining isolates yielded compatible results. In conclusion, susceptibility tests, when performed according to NCCLS recommendations, are found to be reliable and decisive, for the detection of methicillin resistance of S. aureus.