Detection of Methicillin Resistance in Staphylococcus aureus by Disk Diffusion and PCR Methods

Background and Objective: Methicillin resistance in Staphylococcus aureus is an increasingly important clinical problem. A chromosomal gene, mecA, mediates resistance to penicillinase-resistant penicillins such as methicillin and oxacillin in Staphylococcus aureus. We evaluated the validity of disk diffusion test by using oxacillin, methicillin and cefoxitin disks with consideration of the presence of mecA gene as the reference method for detection of methicillin resistant Staphylococcus aureus (MRSA). Materials and Methods: The susceptibility testing of 222 S. aureus clinical isolates to oxacillin (1 ?g), cefoxitin (30 ?g) and methicillin (5 ?g) was carried out by the disk diffusion method according to the Clinical Laboratory Standards Institute guidelines. Detection of mecA gene was performed using PCR method. Results: An amplified mecA gene of 310 bp was detected in 55% of examined strains by PCR, thus 55% strains were considered MRSA. Sensitivity of oxacillin, methicillin and cefoxitin disks were determined 100%, 99.1% and 98.3% respectively. Conclusion: All MRSA strains in PCR had shown resistance to penicillinase-resistant penicillins by oxacillin disk, but two and one strains were sensitive by cefoxitin and methicillin disk respectively. Thus, oxacillin was the most appropriate disk for detecting MRSA. The prevalence of MRSA in this study is comparable to that found in United States, Canada, Europe and Iran, but the percentage of MRSA isolates is almost twice of percentage reported from Japan.