Mannitol Salt Agar-Cefoxitin Combination as a Screening Medium for Methicillin-Resistant Staphylococcus aureus

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Abstract
In disk diffusion tests, cefoxitin is now considered a better indicator than oxacillin for the presence of the mecA gene in Staphylococcus aureus . A logical extension of this work is the incorporation of cefoxitin into media selective for methicillin-resistant Staphylococcus aureus (MRSA). This paper describes the development and subsequent testing of mannitol salt agar containing 4 mg/liter cefoxitin with a unique collection of well-characterized MRSA strains, including low-level methicillin-resistant strains and an equal number of known mecA -negative S. aureus strains. The agar supported the growth of 96.6% of the mecA -positive strains in the collection and inhibited the growth of 100% of the mecA -negative strains. These results suggest that selective media based on cefoxitin are superior to those based on oxacillin for the detection of MRSA.

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