ABSTRACT
BACKGROUND: Methicillin-resistant Staphylococcus aureus (MRSA) has a heteroresistant nature, so methicillin resistance is influenced by various culture conditions, such as temperature, incubation time, and NaCl content in the medium. Mueller Hinton (MH) agar containing 2% NaCl and mannitol salt agar (MSA) with oxacillin disk were evaluated for the detection of methicillin resistance. METHODS: Disk diffusion test on plain Mueller- Hinton (MH) agar, 2% NaCl MH agar, and MSA with 1 microgram oxacillin disk was performed in 70 Stap hylococcus aureus isolates. Oxacillin MIC was determined by E-test. As a gold standard of methicillin resistance, mecA gene was amplified by PCR and detected by agarose gel electrophoresis. RESULTS: Plain MH agar could not detect heterogeneous resistance in 12 S. aureus isolates (18%), but 2% NaCl MH agar and MSA could correctly detect homogeneous and heterogeneous resistance. S. aureus isolates from stool have as much as 48% heterogeneous resistance, while those from non-stool specimen have 5%. CONCLUSION: 2% NaCl and MSA can be used reliably for accurate susceptibility testing of methicillin resistance in routine laboratory.