Use of the polymerase chain reaction for rapid detection of high-level mupirocin resistance in staphylococci

The minimum inhibitory concentrations (MICs) of mupirocin were determined b y the E test (AB Biodisk, Sweden) and the agar dilution method for 107 stap hylococci. The organisms consisted of 34 coagulase-negative staphylococci a nd 73 methicillin-resistant Staphylococcus aureus. Polymerase chain reactio n (PC) primers designed to amplify a 456 bp region of the plasmid-borne iso leucyl tRNA synthetase gene (ileS-2), responsible for high-level mupirocin resistance in staphylococci, were used on DNA preparations from these isola tes. Isolates with high-level mupirocin resistance due to the ileS-2 gene s hould be PCR positive. There was close correlation between the E test and a gar dilution MIC values, with only two strains: differing by more than two serial dilutions. Most (51 of 51 strains) of the high-level resistant strai ns (MIC > 256 mu g/ml) were resistant to the highest concentration of mupir ocin tested (1024 mu g/ml). PCR correctly classified all but four (96%) of the isolates in accordance with the results of agar dilution. All four isol ates 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 mu g/ml; on prolonged incubation they produced halos wi thin the inhibition zone on agar diffusion testing, suggesting that the phe notypic results may have been erroneous. One of 54 isolates for which the M IC exceeded 256 mu g/ml was PCR negative when tested by the original method ology, but a 456 bp product was produced when retested using a lowered anne aling temperature. One isolate for which the MIC of mupirocin was 16 mu g/m l by agar dilution and 8 mu g/ml by the E test was positive by PCR. PCB of the ileS-2 gene is a useful, rapid method for detecting high-level mupiroci n resistance in staphylococci.