Screening method for recovery of methicillin-resistant Staphylococcus aureus from primary plates

A study designed to screen for the presence of methicillin-resistant Staphylococcus aureus from primary plates was conducted from 1 January to 1 September 1985 in a small community hospital. The screening method used a plate of lipovitellin salt mannitol agar and a 4-microgram oxacillin disk incubated at 30 degrees C. Growth of yellow colonies, typical of S. aureus, around the disk without a zone of inhibition was called presumptive methicillin-resistant S. aureus. All susceptibilities were confirmed by using the National Committee for Clinical Laboratory Standards macrodilution technique. Of 224 cultures containing S. aureus, 118 (53%) were positive for methicillin-resistant S. aureus isolates. Of these 118, 111 (94%) were correctly identified from the primary plates as methicillin-resistant S. aureus. Of the 224 isolates, 14 could not be categorized from the primary plates as methicillin-resistant S. aureus due to the small amounts of S. aureus recovered.