Evaluation of the Mastalex latex agglutination test for methicillin resistance in Staphylococcus aureus grown on different screening media

The Mastalex MRSA latex agglutination method was evaluated with 52 methicillin-resistant and 27 methicillin-susceptible strains of Staphylococcus aureus grown on various media. All tests were correct with colonies grown on blood agar with or without oxacillin 2 mg/L. Tests on colonies grown on mannitol salt agar were less reliable, although addition of oxacillin 2 mg/L improved performance. One of the 26 MRSA which grew on Baird-Parker medium with 8 mg/L ciprofloxacin gave a false-negative result. Agglutination was faster when strains were grown on media with oxacillin. The method would be particularly useful for urgent confirmation of resistance.