Mannitol salt agar-cefoxitin combination as a screening medium for methicillin-resistant Staphylococcus aureus

Area of Science:

• Microbiology
• Clinical Diagnostics
• Antimicrobial Resistance

Background:

• Oxacillin disk diffusion is a standard test for detecting methicillin resistance in Staphylococcus aureus.
• The mecA gene confers methicillin resistance in S. aureus.
• Cefoxitin has emerged as a more reliable indicator of the mecA gene than oxacillin.

Purpose of the Study:

• To develop and evaluate a new selective mannitol salt agar incorporating cefoxitin for improved MRSA detection.
• To compare the efficacy of cefoxitin-based media against oxacillin-based methods.

Main Methods:

• Development of mannitol salt agar supplemented with 4 mg/liter cefoxitin.
• Testing the agar with a characterized collection of mecA-positive (including low-level resistant) and mecA-negative Staphylococcus aureus strains.

Main Results:

• The cefoxitin-containing agar supported the growth of 96.6% of mecA-positive MRSA strains.
• The selective media successfully inhibited the growth of 100% of mecA-negative S. aureus strains.

Conclusions:

• Selective media incorporating cefoxitin demonstrate superior performance for MRSA detection compared to oxacillin-based media.
• This cefoxitin-based agar provides a reliable and effective tool for identifying MRSA in clinical settings.