A screening test for the detection of methicillin - resistant staphylococci GILLIAN

Reports of the isolation of methicillin-resistant strains of coagulase-positive Staphylococcus aureus from patients in hospitals have appeared since 1961, and the estimated incidence of such strains is very variable. Recently in this laboratory several strains of Staph. aureus isolated were noted togivea diminished zone of inhibition with a 10 ,tg methicillin disc on routine sensitivity testing. They had the same or closely related phage-typing patterns, belonging to group III. For this reason, subcultures were inoculated into broth containing 12-5 .tg/ml methicillin and incubated for 48 hours at 37°C. All were found to be resistant. Barber (1964) has shown that methicillin-resistant staphylococci will grow normally in the presence ot methicillin on media containing added 5% NaCl, which increases the osmotic pressure of the medium and prevents lysis of the organisms. Sutherland (1964) has demonstrated that these staphylococci will grow up to the edge of a 10 ,ug methicillin disc on this medium and suggested that such a test might be suitable for the routine investigation of strains for methicillin sensitivity. A screening test which utilizes this characteristic of resistant strains is described.