MRSADifferentiationamong 752 S. aureus Strains Using a Cefoxitin Disk

Overview

A set of 752 S.
aureus strains including 665 fresh clinical isolates, 82 collection strains from the NRLfor staphylococci and three control strains for external quality assessment were tested for susceptibilityto oxacillin by three routine phenotypic methods with oxacillin (agar screening method,dilution micromethod and disk diffusion method) and a new method with a 30 µg cefoxitin disk.Gene mecA coding for oxacillin resistance was detected by PCR, PBP2a gene product was detectedby 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 foroxacillin resistant strains, eight out of these strains showing false oxacillin susceptibility in one ormore phenotypic tests. It can be stated that the presence of an inhibition zone of < 20 mm aroundthe 30 µg cefoxitin disk allows for reliable differentiation between MRSA and oxacillin susceptibleS. aureus.

Key words:
MRSA – disk method – cefoxitin.