Comparison of susceptibility testing methods for the detection of methicillin/oxacillin resistance in Staphylococcus aureus

ABSTRACT

To compare the accuracy of disk diffusion method and E-test for the detection of methicillin resistance and low-level methicillin-resistance in Staphylococcus aureus [S. aureus] and the PBP2a latex agglutination test for confirmation. A total of 76 methicillin resistant S. aureus [MRSA] isolates from different clinical specimens were tested by disk diffusion method. Disk d i ffusion method was performed using methicillin [MET] 5microg disk, oxacillin [OX] 1 microg disk, moxalactam [MOX], and cefoxitin [FOX] 30 microg each on Mueller Hinton agar [MHA] plates supplemented with 2% NaCl and incubated at 35 °C for 24 hours. Minimum inhibitory concentration [MIC] was performed by E-test for MET and OX on MHA plates containing 2% NaCl. Results for all tests were read according to NCCLS recommendations for zone of inhibition and break points. Low-level MRSA strains were confirmed by PBP2a latex agglutination test. All strains were tested for ?-lactamase production. All MRSA strains were detected by disk diffusion methods using MET, OX and FOX [100%]. Four [5.2%] strains were low-level MRSA by MOX disk. E-test detected 72 [94.7%] using MET and 74 [97.3%] MRSA strains using OX. No heterogeneous growth within the zones of inhibitions was noticed. One MRSA was misclassified as methicillin sensitive by MET E-test [MIC 6 microg /ml], but was 32 microg/ml by OX E-test. Two strains were low-level MRSA by E-tests but showed resistance by MET, OX and FOX disk diffusion method. One strain had MIC of 12 microg/ml both by OX and MET E-tests. All four strains showed low-level resistance by MOX disk and were positive for PBP2a latex agglutination test. All the strains produced ?-lactamase. Conclusion: Disk diffusion method using MET, OX, and FOX can reliably be used to detect methicillin resistance in S. aureus. MOX and E-test can be used to detect lowlevel methicillin resistance and these can further be confirmed by PBP2a latex agglutination test in diagnostic laboratories