Evaluation of a disk diffusion method with cefoxitin (30 microg) for detection of methicillin-resistant Staphylococcus aureus.

The emergence of heterogeneous populations of methicillin-resistant Staphylococcus aureus (MRSA) causes major problems in routine screening for MRSA. In heterogeneous MRSA populations, a proportion of bacterial cells show low-level resistance to oxacillin, with minimal inhibitory concentrations (MICs) of oxacillin ranging between 1 and 100 mg/l, while in homogeneous MRSA populations, the MIC of oxacillin for all cells is >100 mg/l. Routine oxacillin disk diffusion tests often fail to detect heterogeneous MRSA populations. In the present study, a recently proposed disk diffusion method that employs a cephamycin antibiotic (cefoxitin 30 microg; BD Sensi-disc, Becton Dickinson, Germany) was evaluated using 155 clinical isolates of S. aureus (73 mecA positive and 82 mecA negative). The results were compared with those of other MRSA screening techniques: a disk diffusion test with oxacillin 1 microg and cefoxitin 30 microg (BD Sensi-disc; Becton Dickinson), an MRSA latex agglutination test (Denka Seiken, Japan), and an oxacillin screen agar test (6 microg/ml; Becton Dickinson). Detection of the mecA gene by polymerase chain reaction was considered the gold standard. The performances of the different methods were determined and compared. The results showed that the cefoxitin disk diffusion test is preferable to the oxacillin disk diffusion method for routine screening to detect MRSA.