A Comparison between Polymerase Chain Reaction, Oxacillin Agar Dilusion and Cefoxitin Disk Diffusion Methods in Detection of Methicillin Resistance in Staphylococcus Aureus

Staphylococcus aureus (S. aureus) is an important human pathogen associated with hospital- and community-acquired diseases، a wide range of infectious disease including minor skin disease to more sever and aggressive infections such as septicemia، pneumonia، endocarditis، and deep skin abscess. S. aureus has a high mortality rate of 35%. Therefore، it is crucial to identify the methicillin resistant S. aureus (MRSA) strains and implement the necessary treatment in order to control the spread of hospital infections. A total of 150 S. aureus isolates were collected from samples of patients admitted to Alzahra and Shariati hospitals in Isfahan، Iran. Polymerase chain reaction (PCR) for mecA gene was performed in all strains. Thereafter، the minimum inhibitory concentration (MIC) for oxacillin was carried out using agar dilution method. Then، antibiogram using disk diffusion for cephoxitin was performed according to Clinical and Laboratory Standards Institute (CLSI) regulations on isolates containing mecA gene. The three methods were then compared. Of the 150 samples، 62 samples (41. 33%) were found to carry mecA gene using PCR. However، agar dilution for oxacillin found 56 of 62 samples (90. 33%) to harbor mecA gene. Disk diffusion method revealed that 53 of 62 samples (85. 48%) contain mecA gene. Agar dilution method was found to have a higher sensitivity in determining antibiotic sensitivity compared to disk diffusion method. However، PCR was identified as the ideal method for detecting MRSA strains; since a number of strains were found to be sensitive to oxacillin in phenotypic test due to lack of mecA expression while still containing the gene.