A comparison of phenotypic methods in detecting methicillin resistance in staphylococcus aureus

Staphylococci are the major human pathogens that cause hospital and communityacquired infections.MRSA strains produce a penicillin binding protein named PBP2a, which presentslow binding affinity to ß-laktam antibiotics. This protein is encoded by thechromosomal mecA gene. S.aureus strains that involve mecA gene and PBP2a conferresistance to all ß-laktam antibiotics. Methicillin resistance coded by mecA geneemerges as homogeneous or heterogeneous resistance. The phenotypic expression ofmethicillin resistance in heteroresistant strains occurs only in some cells in the givenpopulation. In addition to mecA gene, other genetic determiners in different parts arealso responsible for the heterogeneous expression of the resistance (28).It is not always possible to accurately detect methicillin resistance in Staphylococcusaureus strains with phenotypic methods due to its heteroresistant characteristic.This study analyzed 66 MRSA strains isolated from the samples of the hospitalizedpatients in Gazi University Medical Hospital. In the MRSA starins analyzed, thepresence of mecA gene is first detected with PCR method, and then oxacillincefoxitin disk diffusion, oxacillin agar screen, oxacillin cefoxitin E test and PBP2alatex agglutination methods are compared in terms of their sensitivity in detectingmethicillin resistance. The phenotypic methods used in this study were found to beequally sensitive (98,48%) in detecting MRSA strains. Only one strain does notconfer methicillin resistance phenotypically with these methods although the isolatehas mecA gene.It is concluded in this study that in cases where molecular techniques cannot beapplied, oxacillin cefoxitin disk diffusion, oxacillin agar screen, oxacillin cefoxitin Etest and PBP2a latex agglutination methods can be used with equal sensitivity. Theresults of this study show that none of the phenotypic methods is 100% sensitive.However, if PCR method that detects mecA gene is not available, cefoxitin diskdiffusion is suitable for MRSA detection due to the fact that it is accepted as areference method in various studies and can be applied easily.