Abstract
Testing of staphylococci other than Staphylococcus aureus (SOSA) for mecA-mediated resistance is challenging. Isolates of Staphylococcus capitis, Staphylococcus haemolyticus, Staphylococcus hominis, and Staphylococcus warneri were evaluated by cefoxitin and oxacillin broth microdilution (BMD), disk diffusion (DD), and PBP2a immunoassay, and the results were compared to mecA PCR results. No phenotypic susceptibility test correlated well with PCR results across all species, although the PBP2a immunoassay yielded 100% correlation. Oxacillin BMD testing by current Clinical and Laboratory Standards Institute (CLSI) SOSA breakpoints led to 2.1% very major errors (VMEs) and 7.1% major errors (ME). Adjusting this breakpoint up by a dilution (susceptible, ≤0.5 μg/ml; resistant, ≥1.0 μg/ml) led to 2.8% VMEs and 0.3% MEs. Among species evaluated, S. haemolyticus had unacceptable VMEs with this new breakpoint (6.4%), as did S. hominis (4.0%). MEs were acceptable by this new breakpoint, ranging from 0 to 1.2%. Oxacillin DD yielded high ME rates (20.7 to 21.7%) using CLSI or European Committee on Antimicrobial Susceptibility Testing breakpoints. VMEs ranged from 0 to 5.3%. Cefoxitin BMD led to 4.9% VMEs and 1.6% MEs. Cefoxitin DD performed best when interpreted with the CLSI SOSA breakpoint, with 1.0% VMEs and 2.9% MEs. This study led CLSI to adjust the oxacillin MIC breakpoints for SOSA. Laboratories should be aware that no individual phenotypic test correlates well across all species of SOSA with mecA PCR results. Molecular testing for mecA or evaluation for PBP2a is the preferred approach.
Highlights
Testing of staphylococci other than Staphylococcus aureus (SOSA) for mecA-mediated resistance is challenging
The Clinical and Laboratory Standards Institute (CLSI) AST Subcommittee agreed that implementation of species-specific breakpoints for SOSA may be unrealistic for laboratories, given the complexity of testing and the fact that not all laboratories can accurately perform identification of these isolates to the species level
While routine identification of SOSA to the species level may not be indicated in all cases, we believe species identification should be attempted for susceptibility testing when isolates are deemed clinically relevant, such as repeated positive blood cultures or recovery from infected prostheses
Summary
Testing of staphylococci other than Staphylococcus aureus (SOSA) for mecA-mediated resistance is challenging. No phenotypic susceptibility test correlated well with PCR results across all species, the PBP2a immunoassay yielded 100% correlation. Oxacillin BMD testing by current Clinical and Laboratory Standards Institute (CLSI) SOSA breakpoints led to 2.1% very major errors (VMEs) and 7.1% major errors (ME). Adjusting this breakpoint up by a dilution (susceptible, Յ0.5 g/ml; resistant, Ն1.0 g/ml) led to 2.8% VMEs and 0.3% MEs. Among species evaluated, S. haemolyticus had unacceptable VMEs with this new breakpoint (6.4%), as did S. hominis (4.0%). Laboratories should be aware that no individual phenotypic test correlates well across all species of SOSA with mecA PCR results. Molecular testing for mecA or evaluation for PBP2a is the preferred approach
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