Comparison of phenotypic versus genotypic methods in the detection of methicillin resistance in Staphylococcus aureus.

BACKGROUND & OBJECTIVE Conventional methods to detect methicillin resistance in Staphylococcus aureus are inadequate as expression of resistance is subject to environmental and conditional expression of PBP2a antigen. The objective of the present study was to determine methicillin resistance in S. aureus by conventional susceptibility (oxacillin disc diffusion and oxacillin MIC) and molecular methods (PCR) and to evaluate latex agglutination test for the detection of PBP 2a and to compare the results of these tests for its sensitivity, specificity and rapidity. METHODS A total of 150 consecutive clinical isolates of Staphylococcus aureus received at the Department of Microbiology, Christian Medical College, Vellore, were included. Oxacillin (1 mg) disc diffusion and agar dilution method were used. The isolates were also subjected to latex agglutination test for detection of PBP2a and multiplex PCR to detect mecA and femB genes. RESULTS Of the 150 isolates, 33 were found to be MRSA by oxacillin disc diffusion. By MIC method, 13 per cent of the isolates had values 32 microg/ml, 6 per cent between 16-8 mug/ml and 2.7 per cent had a value of 4 microg/ml; 100 per cent concordance was obtained between the oxacillin disc screening and MIC methods. The latex agglutination showed positive reaction for all MRSA with only one MSSA being falsely classified as MRSA. The specificity and sensitivity were 99 and 100 per cent respectively. Test results were obtained within 15 min. By multiplex PCR, all 22 per cent of MRSA were positive for mecA and femB genes and additionally one MSSA carried mecA gene. However, femB gene was not found in 6 MSSA isolates. Specificity and sensitivity of PCR for mecA detection was similar to latex agglutination test. PCR system required approximately five hours. INTERPRETATION & CONCLUSION Our findings showed that the conventional methods for detection of methicillin resistance like disc screening, disc diffusion and MIC are cost-effective but time consuming. Latex agglutination though expensive is rapid and can be a good preliminary screen with high sensitivity and specificity. Multiplex PCR is a good confirmatory test though expensive.