Comparison of cefoxitin disc diffusion test, oxacillin screen agar, and PCR for mecA gene for detection of MRSA

Cefoxitin is a potent inducer of the mecA regulatory system. It is being recommended for detection of methicillin resistance in Staphylococcus aureus (MRSA) when using disk diffusion testing. The aim of our study was to evaluate the efficacy of cefoxitin disc diffusion test to characterize MRSA and compare it with oxacillin agar screening and detection of mecA gene by PCR.

Fifty strains of S. aureus isolated from clinical samples were used in the study. Routine antibiotic susceptibility testing was performed including oxacillin disk. Oxacillin screen agar plates with 4% NaCl and 6 microg/ml of oxacillin were inoculated and interpreted as per standard guidelines. Cefoxitin disc diffusion test was performed using 30 microg disc and zone sizes were measured. PCR for amplification of the mecA gene was performed.

Out of the 50 isolates, 28 were found to be methicillin resistant by oxacillin disc diffusion test, 30 were resistant by oxacillin screen agar method, and 32 were resistant with cefoxitin disc diffusion. For these 32 isolates mecA gene was positive.

Results of cefoxitin disc diffusion test is in concordance with the PCR for mecA gene. Thus, the test can be an alternative to PCR for detection of MRSA in resource constraint settings.