| Abstract: | Mupirocin susceptibility testing of Staphylococcus aureus has become more important as mupirocin is used more widely to suppress or eliminate S. aureus colonization and prevent subsequent health care- and community-associated infections. The present multicenter study evaluated two susceptibility testing screening methods to detect mupirocin high-level resistance (HLR), broth microdilution (BMD) MICs of ≥512 μg/ml, and a 6-mm zone diameter for a disk diffusion (DD) test with a 200-μg disk. Initial testing indicated that with Clinical and Laboratory Standards Institute methods for BMD and DD testing, the optimal conditions for the detection of mupirocin HLR were 24 h of incubation and reading of the DD zone diameters with transmitted light. Using the presence or absence of mupA as the “gold standard” for HLR, the sensitivity and specificity of a single-well 256 μg/ml BMD test were 97 and 99%, respectively, and those for the 200-μg disk test were 98 and 99%, respectively. Testing with two disks, 200 μg and 5 μg, was evaluated for its ability to distinguish HLR isolates (MICs ≥ 512 μg/ml), low-level-resistant (LLR) isolates (MICs = 8 to 256 μg/ml), and susceptible isolates (MICs ≤ 4 μg/ml). Using no zone with both disks as an indication of HLR and no zone with the 5-μg disk plus any zone with the 200-μg disk as LLR, only 3 of the 340 isolates were misclassified, with 3 susceptible isolates being classified as LLR. Use of standardized MIC or disk tests could enable the detection of emerging high- and low-level mupirocin resistance in S. aureus.Mupirocin is a topical antibacterial agent that is used both for the treatment of skin infections and for the suppression or elimination of nasal carriage of Staphylococcus aureus, including methicillin-resistant S. aureus (MRSA) (8). The recommendations of the Healthcare Infection Control Practices Advisory Committee suggest the use of a tiered approach to the prevention and control of infections with multidrug-resistant organisms, including MRSA, in acute-care settings (20). In their recommendations, decolonization is presented as one intervention that may be considered when intensified MRSA control measures are needed; if decolonization is used, susceptibility testing and monitoring for the emergence of resistance to the decolonization agent are recommended in one study (21).There are two levels of resistance to mupirocin: low-level resistance (LLR), for which the MICs are 8 to 256 μg/ml, and high-level resistance (HLR), for which the MICs are ≥512 μg/ml (11). The mupirocin MICs of strains susceptible to mupirocin are MICs ≤4 μg/ml. HLR is associated with the presence of the plasmid-mediated mupA gene, which encodes a mupirocin-resistant isoleucyl-tRNA synthetase, although S. aureus strains with HLR that lack mupA have occurred (this study) and can also be created in the laboratory (23). LLR results from mutation of the native, chromosomal isoleucyl-tRNA synthetase ileS gene (1). Studies suggest that S. aureus strains with HLR to mupirocin cannot be successfully eliminated with mupirocin and that the occurrence of HLR is increasing (22). It has been suggested that S. aureus strains demonstrating LLR could be eliminated by topical application of mupirocin because of the high concentrations achieved locally, but this has not been demonstrated definitively (11, 21).Until recently, methods for testing topical agents have not been included in susceptibility testing documents published by the Clinical and Laboratory Standards Institute (CLSI; formerly NCCLS), although guidelines for testing by various methods have been suggested by others (9, 10, 12, 13, 16, 17). The British Society for Antimicrobial Chemotherapy has formal recommendations for the testing of mupirocin (www.bsac.org.uk) that include testing of a 5-μg and a 20-μg mupirocin disk. Their recommendations require MIC testing to determine the level of resistance if a 5-μg disk is used alone. An initial investigation at the Centers for Disease Control and Prevention (CDC), Atlanta, GA, showed that a 200-μg mupirocin disk was able to differentiate isolates with LLR from those with HLR (15). We undertook the study described here to determine the MIC and disk diffusion criteria for the detection of S. aureus strains with high- or low-level mupirocin resistance and to validate quality control tests. Using data from this study, a screen test for prediction of high-level mupirocin resistance is now included in CLSI susceptibility testing documents (3, 6, 7). |
