SPECIES-SPECIFIC IDENTIFICATION OF METHICILLIN RESISTANCE IN STAPHYLOCOCCI

The ability to identify methicillin-resistant staphylococci by the dis c diffusion method was evaluated using discs containing oxacillin (1, 5 and 10 mu g), methicillin (10 Gig) and cephalexin (30 Gig) Strains o f Staphylococcus aureus (67 strains) and coagulase-negative staphyloco cci (72 novobiocin-sensitive and 27 novobiocin-resistant strains) were studied using two inoculum densities (10(6) cfu/ml and 10(8) cfu/ml). Inhibitory zones were recorded after 18, 24 and 42 hours of incubatio n. A mecA-specific application of the polymerase chain reaction was us ed as a reference method. The inoculum of 10(8) cfu/ml and incubation for 24 hours were optimal for the identification of methicillin-resist ant strains. However, one single disc was not sufficient for the ident ification of methicillin resistance in the different staphylococcal sp ecies. The mecA-positive strains of Staphylococcus aureus and novobioc in-resistant coagulase-negative species were clearly separated from th e mecA-negative strains when the 5 mu g oxacillin disc was used, where as the 1 mu g oxacillin disc was optimal for the identification of the mecA-positive novobiocin-sensitive coagulase-negative strains.