Discrepancies between mecA PCR and conventional tests used for detection of methicillin resistant Staphylococcus aureus

Conventional and molecular techniques are being used in the detection of me thicillin resistance in Staphylococcus aureus but they do not always show c oncordant results. In this study, a mecA PCR-based amplification was compar ed with the 1 mu g oxacillin disk diffusion test and the Epsilometer test ( E-test) for detection of MICs. Among 31 isolates initially characterized as MRSA by the disk diffusion test, mecA was detected in only 13 (42%) isolat es. The E-test showed a wide range of oxacillin MICs (0.5 - > 256 mu g/ml) among these 31 MRSA isolates: seven isolates had an MIC of > 256 mu g/ml, o ne had 64 mu g/ml, two had 4 mu g/ml, two had 3 mu g/ml, one had 2.5 mu g/m l, nine had 2 mu g/ml, three had 1.5 mu g/ml, five had 1 mu g/ml and one ha d 0.5 mu g/ml. Comparing the mecA PCR results with the E-test oxacillin MIC findings revealed that mecA was detected in seven of eight isolates (87.5% ) with an MIC of greater than or equal to 64 mu g/ml, in three of 14 isolat es (21.4%) with an MIC of 2-4 mu g/ml and in three of nine isolates (33.3%) with an MIC of < 2 mu g/ml. beta-lactamase production was positive in 28/3 1 isolates (90.3%). Because of this variation between tests and since sever al resistance mechanisms are known to mediate methicillin resistance in S. aureus, the reliable detection of MRSA cannot be solely based on detection of mecA gene in S. aureus. At this stage and until new guidelines are intro duced by an official body, such as NCCLS, a combination of conventional met hods alone or together with a molecular method should be used every time S. aureus is tested for detection of methicillin resistance. (C) 1999 Elsevie r Science B.V. and International Society of Chemotherapy. All rights reserv ed.