Multicenter study to determine disk diffusion and broth microdilution criteria for prediction of high- and low-level mupirocin resistance in Staphylococcus aureus

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 >or=512 microg/ml, and a 6-mm zone diameter for a disk diffusion (DD) test with a 200-microg 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 microg/ml BMD test were 97 and 99%, respectively, and those for the 200-microg disk test were 98 and 99%, respectively. Testing with two disks, 200 microg and 5 microg, was evaluated for its ability to distinguish HLR isolates (MICs >or= 512 microg/ml), low-level-resistant (LLR) isolates (MICs = 8 to 256 microg/ml), and susceptible isolates (MICs <or= 4 microg/ml). Using no zone with both disks as an indication of HLR and no zone with the 5-microg disk plus any zone with the 200-microg 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.

FIG. 1.

Scatter plot of broth microdilution MICs obtained with Difco CAMHB and disk diffusion zone diameters obtained with BD 200-μg disks after repeat testing of discrepant strains identified during phase 2. mupA-positive strains are marked with an asterisk and are presented slightly off center of the mupA-negative strains. See the text for a description and explanation of the discrepancies.

FIG. 2.

Scatter plot of the broth microdilution MICs obtained with Difco CAMHB and disk diffusion zone diameters obtained with BD 5-μg disks after repeat testing of discrepant strains identified during phase 2. mupA-positive strains are marked with an asterisk and are presented slightly off center of the mupA-negative strains. The horizontal line separates mupirocin-susceptible isolates (MICs ≤ 4 μg/ml) from isolates that are mupirocin nonsusceptible. The vertical line is a proposed breakpoint that would separate mupirocin-susceptible isolates (zone diameters, ≤11 mm) from nonsusceptible isolates using the disk diffusion test with the 5-μg disk.