Journal of Antimicrobial Chemotherapy, Vol 40, 241-249, Copyright © 1997 by The British Society for Antimicrobial Chemotherapy
JO Jarlov, C Busch-Sorensen, F Espersen, I Mortensen, DM Hougaard and VT Rosdahl
The efficacy of 19 agar diffusion methods for the detection of methicillin
resistance among coagulase-negative staphylococci (CoNS) within 24 h was
evaluated. A total of 359 CoNS isolates were tested, of which 204 were
Staphylococcus epidermidis. In 164 isolates, the presence of mecA was
investigated; 61 strains were mecA-positive and 103 were mecA-negative by
Southern blot analysis. Based on the best agreement shown with the mecA
determination (94%) among four agar dilution assays for determining
methicillin MIC, an assay with Columbia agar supplemented with NaCl and
incubation with a heavy bacterial inoculum of 10(5)-10(6) cfu/spot was used
as the reference MIC method. The best agar diffusion results were obtained
with a 1 microg oxacillin disc on Columbia agar with 4.5% NaCl supplement.
With this method, 99% of S. epidermidis and 94% of non-S. epidermidis were
in agreement with the MIC determination. However, Columbia (without NaCl),
Mueller-Hinton and Isosensitest agars were almost as useful when a 1 microg
oxacillin disc was used. The zone breakpoints for S. epidermidis were, in
general, considerably larger than those for other CoNS species and,
consequently, differentiation according to species is recommended.
Furthermore, resistance to other antibiotics, such as gentamicin and
erythromycin, makes methicillin resistance highly likely.
ORIGINAL ARTICLES
Evaluation of different methods for the detection of methicillin resistance in coagulase-negative staphylococci [published erratum appears in J Antimicrob Chemother 1998 Feb;41(2):317-8]
Department of Clinical Microbiology, National University Hospital (Rigshospitalet), Copenhagen, Denmark.
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