Use of the Polymerase Chain Reaction for Rapid Detection of High-Level Mupirocin Resistance in Staphylococci |
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Authors: | R M Anthony A M Connor E G M Power G L French |
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Institution: | (1) Department of Microbiology, University Medical and Dental School, St. Thomas' Hospital, Lambeth Palace Road, London SE1 7EH, UK, GB |
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Abstract: | The minimum inhibitory concentrations (MICs) of mupirocin were determined by the E test (AB Biodisk, Sweden) and the agar
dilution method for 107 staphylococci. The organisms consisted of 34 coagulase-negative staphylococci and 73 methicillin-resistant
Staphylococcus aureus. Polymerase chain reaction (PCR) primers designed to amplify a 456 bp region of the plasmid-borne isoleucyl tRNA synthetase
gene (ileS–2), responsible for high-level mupirocin resistance in staphylococci, were used on DNA preparations from these isolates.
Isolates with high-level mupirocin resistance due to the ileS–2 gene should be PCR positive. There was close correlation between the E test and agar dilution MIC values, with only two
strains differing by more than two serial dilutions. Most (51 of 54 strains) of the high-level resistant strains (MIC>256 μg/ml)
were resistant to the highest concentration of mupirocin tested (1024 μg/ml). PCR correctly classified all but four (96%)
of the isolates in accordance with the results of agar dilution. All four isolates that gave discrepant results were methicillin-resistant
Staphylococcus aureus. Two of these were PCR positive, yet the MIC of mupirocin for these strains was <0.06 μg/ml; on prolonged incubation they
produced halos within the inhibition zone on agar diffusion testing, suggesting that the phenotypic results may have been
erroneous. One of 54 isolates for which the MIC exceeded 256 μg/ml was PCR negative when tested by the original methodology,
but a 456 bp product was produced when retested using a lowered annealing temperature. One isolate for which the MIC of mupirocin
was 16 μg/ml by agar dilution and 8 μg/ml by the E test was positive by PCR. PCR of the ileS–2 gene is a useful, rapid method for detecting high-level mupirocin resistance in staphylococci. |
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