Comparison of Susceptibility Testing Methods for the Detection of Methicillin/Oxacillin Resistance in Staphylococcus Aureus

Objective: 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. Materials and Methods: 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) 5μg disk, oxacillin (OX) 1 μg disk, moxalactam (MOX), and cefoxitin (FOX) 30 μg each on Mueller Hinton agar (MHA) plates supplemented with 2% NaCl and incubated at 35 oC 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 re c o m m e n d a t i o n s for zone of inhibition and break points. Low-level MRSA strains were confirmed by PBP2a latex agglutination test. All strains were tested for ß-lactamase production. R e s u l t s : All M R S A 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 M R S A w a s misclassified as methicillin sensitive by MET E-test (MIC 6 μg /ml), but was 32 μg/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 μg/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 ß-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.