[Detection of MRSA in a group of 752 strains of S. aureus using a cefoxitin disk]

A set of 752 S. aureus strains including 665 fresh clinical isolates, 82 collection strains from the NRL for staphylococci and three control strains for external quality assessment were tested for susceptibility to oxacillin by three routine phenotypic methods with oxacillin (agar screening method, dilution micromethod and disk diffusion method) and a new method with a 30 micrograms cefoxitin disk. Gene mecA coding for oxacillin resistance was detected by PCR, PBP2a gene product was detected by latex agglutination. All of 218 oxacillin resistant strains--MRSA (methicillin resistant S. aureus)--gave inhibition zones of 6-19 mm around the cefoxitin disk, i.e. zones within the range set up for oxacillin resistant strains, eight out of these strains showing false oxacillin susceptibility in one or more phenotypic tests. It can be stated that the presence of an inhibition zone of < 20 mm around the 30 micrograms cefoxitin disk allows for reliable differentiation between MRSA and oxacillin susceptible S. aureus.