Korean J Infect Dis.  1998 Dec;30(6):539-544.

Detection of Methicillin Resistance in Staphylococcus aureus by 2% NaCl Mueller-Hinton Agar and Mannitol Salt Agar

• Lee CK
• Ma KR
• Lee DH
• Whang SC
• Kim YK
• Lee KN

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.

MeSH Terms

Agar*
Diffusion
Electrophoresis, Agar Gel
Mannitol*
Methicillin Resistance*
Methicillin*
Methicillin-Resistant Staphylococcus aureus
Oxacillin
Polymerase Chain Reaction
Staphylococcus aureus*
Staphylococcus*
Agar
Mannitol
Methicillin
Oxacillin