Comparison of cefoxitin disk diffusion and PCR for mecA gene methods for detection of methicillin resistant Staphylococcus aureus

Background and Objective: Methicillin-resistant Staphylococcus aureus (MRSA) is one of the most common microorganisms that is recovered from clinical bacterial isolates. Clinical and Laboratory Standards Institute (CLSI) recommends usage of cefoxitin when using the disk diffusion method to determine resistance against methicillin for S. aureus. The aim of our study was to evaluate the efficacy of cefoxitin disk diffusion test to detect MRSA and compare it with PCR methods for detection of mecA gene.

Materials and Methods: S. aureus was isolated from patients in five selected hospitals of Tehran province and identified by biochemical conventional methods and PCR for nuc gene (specific for this bacterium). Cefoxitin disc diffusion test was performed using 30 µg disc according to recommendations of CLSI. PCR for amplification of the mecA gene was performed.

Results: With phenotypic methods and PCR for nuc gene, identification of 101 isolates was confirmed as S. aureus. Disk diffusion method showed that all isolates were susceptible to vancomycin and linezolid antibiotics and 58 isolates were found to be methicillin resistant by cefoxitin disc diffusion. For these 58 isolates, mecA gene was positive.

Conclusion: In the absence of availability of molecular biology techniques, the cefoxitin disc can be an alternative to PCR for detection of MRSA as a simple and economically low cost method.