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 p resent study, various conventional methods were compared with polymerase ch ain reaction on 106 clinical isolates of Staph. aureus for detection of oxa cillin resistance. Methods: A total of 106 clinical isolates of Staph. aureus were tested for oxacillin resistance by disc diffusion, screen agar plates (3 mug and 6 mug /ml of oxacillin), oxacillin broth (3 mug/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 me cA negative strains as oxacillin resistant. Screen agar 3 mug, screen agar 6 mug and oxacillin broth 3 mug 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 s ensitivity, specificity and accuracy of disc diffusion, 3 mug screen agar, 6 mug screen agar and 3 mug oxacillin broth against PCR as gold standard we re as follows: 87.7, 89.8 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 dif fusion test was least reliable and 3 mug: broth had the highest sensitivity and specificity when compared with PCR for detection of oxacillin resistan ce. Because of variations among the methods, a combination of tests should be used for the accurate detection of MRSA till new guidelines by an approp riate body are formulated.