Comparison of various conventional methods with a polymerase chain reaction assay for detecting methicillin-resistant & susceptible Staphylococcus aureus strains.

BACKGROUND & OBJECTIVES: Methicillin-resistant Staphylococcus aureus (MRSA), a major nosocomial pathogen world-wide, is often difficult to detect due to the heterogeneous nature of expression of oxacillin resistance. In the present study, various conventional methods were compared with polymerase chain reaction on 106 clinical isolates of Staph. aureus for detection of oxacillin resistance.

METHODS

A total of 106 clinical isolates of Staph. aureus were tested for oxacillin resistance by disc diffusion, screen agar plates (3 micrograms and 6 micrograms/ml of oxacillin), oxacillin broth (3 micrograms/ml) and mecA based PCR.

RESULTS

PCR detected mecA gene amplified product of 604 bp in 57 strains. Disc diffusion failed to detect 7 mecA positive strains but identified 5 mecA negative strains as oxacillin resistant. Screen agar 3 micrograms, screen agar 6 micrograms and oxacillin broth 3 micrograms detected 55, 53 and 55 respectively of the 57 mecA positive strains; however, they also falsely identified 5, 3 and 3 strains of mecA negative strains respectively as oxacillin resistant. The sensitivity, specificity and accuracy of disc diffusion, 3 micrograms screen agar, 6 micrograms screen agar and 3 micrograms oxacillin broth against PCR as gold standard were as follows: 87.7, 89.9 and 88.7 per cent; 96.5, 89.8 and 93.4 per cent; 93.0, 93.9 and 93.4 per cent; 96.5, 93.9 and 95.3 per cent respectively.

INTERPRETATION & CONCLUSIONS: The present study demonstrated that disc diffusion test was least reliable and 3 micrograms broth had the highest sensitivity and specificity when compared with PCR for detection of oxacillin resistance. Because of variations among the methods, a combination of tests should be used for the accurate detection of MRSA till new guidelines by an appropriate body are formulated.