Evaluation of Surrogate Tests for the Presence of -Mediated Methicillin Resistance in Staphylococcus capitis, Staphylococcus haemolyticus, Staphylococcus hominis, and Staphylococcus warneri.

TitleEvaluation of Surrogate Tests for the Presence of -Mediated Methicillin Resistance in Staphylococcus capitis, Staphylococcus haemolyticus, Staphylococcus hominis, and Staphylococcus warneri.
Publication TypeJournal Article
Year of Publication2020
AuthorsHumphries RM, Magnano P, Burnham CA, Bard JDien, Dingle TC, Callan K, Westblade LF
JournalJ Clin Microbiol
Volume59
Issue1
Date Published2020 12 17
ISSN1098-660X
KeywordsAnti-Bacterial Agents, Bacterial Proteins, Cefoxitin, Humans, Methicillin Resistance, Microbial Sensitivity Tests, Oxacillin, Penicillin-Binding Proteins, Staphylococcal Infections, Staphylococcus, Staphylococcus capitis, Staphylococcus haemolyticus, Staphylococcus hominis
Abstract

Testing of staphylococci other than (SOSA) for -mediated resistance is challenging. Isolates of , , , and were evaluated by cefoxitin and oxacillin broth microdilution (BMD), disk diffusion (DD), and PBP2a immunoassay, and the results were compared to 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, had unacceptable VMEs with this new breakpoint (6.4%), as did (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 PCR results. Molecular testing for or evaluation for PBP2a is the preferred approach.

DOI10.1128/JCM.02290-20
Alternate JournalJ Clin Microbiol
PubMed ID33115842
PubMed Central IDPMC7771434
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