Evaluation of different primers for detecting mecA gene by PCR in comparison with phenotypic methods for discrimination of methicillin-resistant Staphylococcus aureus.

Detection of the mecA gene by polymerase chain reaction (PCR) is the gold standard for identifying methicillin-resistant Staphylococcus aureus (MRSA). PCR assays, employing MR1-MR2 primers (primer set 1) and MR3-MR4 primers (primer set 2) to generate 154 and 533 bp fragment, respectively, are most widely used for amplification of mecA gene. The purpose of this study was to evaluate the presence of mecA gene in 100 clinical isolates of S. aureus using PCR with the two pairs of primers. The results were compared to the broth dilution MIC method, oxacillin salt screening method (OSS) and oxacillin disk agar diffusion method (ODD). Fifteen of the 100 isolates showed a discrepancy between the mecA primer sets 1 and 2. Three isolates (3%) without the mecA gene showed discrepancies with phenotypic methods. The sensitivity, specificity and positive and negative predictive values for the 154 and 533 bp products of mecA were 79, 85, 83, 81 and 94, 100, 100, 94%, respectively. The results indicated that primer set 2 was more appropriate than primer set 1 for the detection of mecA gene in MRSA. There was a good correlation among the mecA gene detection, ODD and OSS methods. The discrepancy of three isolates between PCR and phenotypic methods should be clarified for other resistant mechanisms.