Detection of low-level oxacillin resistance in mecA-positive Staphylococcus aureus.
Detection of low-level oxacillin-resistant Staphylococcus aureus is a problem that needs special attention, particularly in relation to methicillin-resistant S. aureus (MRSA) strains in the community that belong to clonal lineage ST80. This study compared different phenotypic methods for the detection of 74 low-level oxacillin-resistant S. aureus strains (oxacillin MIC <or=1 mg/L), 46 MRSA strains (oxacillin MIC >or=2 mg/L) and 117 methicillin-susceptible S. aureus strains. Determination of microbroth dilution MICs for oxacillin was wholly unsatisfactory, and gave a limited specificity for cefoxitin. The sensitivity of disk-diffusion performed according to CLSI recommendations was 92% with an oxacillin 1-microg disk, and 96% with a cefoxitin 30-microg disk; use of a 10-microg cefoxitin disk and a semi-confluent inoculum (breakpoint for resistance <18 mm zone diameter) gave a sensitivity of 97%. When disk-diffusion was performed on IsoSensitest agar with a zone diameter breakpoint for resistance of <22 mm (as recommended by the Swedish Reference Group for Antibiotics), the sensitivity was 95%.
Witte
W
W
Robert Koch Institute, Wernigerode, Germany. wittew@rki.de
Pasemann
B
B
Cuny
C
C
D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000032:analysis*; D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests; D010068:Oxacillin / Q000494:pharmacology*; D046915:Penicillin-Binding Proteins; D010641:Phenotype; D013211:Staphylococcus aureus / Q000187:drug effects*
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D046915:Penicillin-Binding Proteins; C487789:mecA protein, Staphylococcus aureus; D010068:Oxacillin