Abstract

Coagulase negative staphylococci are increasingly found to be involved in nosocomial infections, especially after invasive surgery. Since the prevalence of methicillin resistance among these strains is high, there is a need for a fast and reliable method to detect this resistance. In this investigation 40 CNS strains, consisting of S.epidermidis and S.haemolyticus, were tested using the E-test and the agardilution test. The tests were carried out with methicillin and oxacillin on Mueller Hinton agar, Columbia and PDM agar supplied with 0, 2 and 5% sodium chloride, inocula of 106 en 108 cfu/ml and MIC reading after 24 and 48 h incubation. The presence of the mecA gene, the structural gene for PBP2a, responsible for methicillin resistance in staphylococcus species, was determined by a PCR method and served as reference (golden standard) for methicillin resistance of the isolates. The most reliable results to detect methicillin resistance in these coagulase negative staphylococcus strains were obtained with the E-test as well as the agar dilution test on Mueller Hinton agar supplemented with 2% NaCl, an inoculum of 108 cfu/ml, an incubation temperature of 37 degrees C and an incubation period of 24 h or 48 h (if sensitive after 24 h). The test was more reliable with oxacillin than with methicillin, particularly in the E-test.