Detecting methicillin resistance in staphylococcus aureus: Comparison of different phenotypic methods and the polymerase chain reaction

Cefoxitin is a more potent inducer of the mecA regulatory system compared to oxacillin in Staphylococcus aureus. It has been recommended for the detection of methicillinresistant Staphylococcus aureus (MRSA) when using diskdiffusion testing. The aim of this study is to compare the results of cefoxitin and oxacillin disk-diffusion methods against the polymerase chain reaction (PCR) for the detection of MRSA. A total of 75 strains of S. aureus isolated from different clinical specimens (e.g., pus, blood, wound swabs, tracheal aspirates, eye swabs and cerebrospinal fluid [CSF]) were screened for methicillin resistance by PCR and cefoxitin and oxacillin disk-diffusion tests. The antibiotic susceptibility testing was performed by the Baur disk-diffusion method using oxacillin (1 μg) and cefoxitin (30 μg) disks on Mueller Hinton agar (MHA), and zone diameters as recommended by CLSIs were read at 18 h and 24 h. All MRSA isolates detected phenotypically were confirmed by PCR for the amplification of mecA and nucA genes. Of the 75 isolates screened, 27 were resistant to oxacillin, 30 were resistant to cefoxitin using the diskdiffusion method, while 30 isolates were confirmed as MRSA by PCR. Performing the cefoxitin disk-diffusion method using a 30 μg disk could be a reliable and more accurate method to detect methicillin resistance in S. aureus strains in situations where mecA PCR cannot be performed.