Accuracy of phenotypic methods in detection of methicillin resistant staphylococci species compared to PCR assay for mecA gene

Considering the increasing incidence of infection due to methicillin resistant [MR] staphylococci; reliable, accurate and rapid testing for MR is essential for both antibiotic therapy and infection control regimens. Our mmain objective was to evaluate the accuracy of cefoxitin and moxalactam disk diffusion [DD] tests and that of Mannitol Salt agar [MSA] screening test using cefoxitin and oxacillin for detection of MR among S.aureus and coagulase negative staphylococci [CoNS] using PCR for mecA gene as "gold standard" comparison assay. Commercial MRSA-screen latex agglutination test [LAT] was performed to detect mecA gene product [PBP2a]. The following susceptibility tests were studied: E test for oxacillin minimal inhibitory concentration [MIC], DD test using oxacillin [1[micro]g], cefoxitin [30[micro]g] and moxalactam [30[micro]g] disks on Muller Hinton agar plates and agar screening test using MSA containing oxacillin [MSA-Ox] [1[micro]g, 4[micro]g and 6[micro]g/ml] and MSA containing cefoxitin [MSA-Cefox] [4[micro]g/ml]. Methicillin resistance was detected in 74% of 77 staphylococcal clinical isolates [68.8% in 48 S.aureus and 82.7% in 29 CoNS]. Compared to PCR assay, LAT showed 100% sensitivity and specificity for both S.aureus and CoNS isolates. Among S.aureus isolates, diagnosis of MR was best achieved by E-test, moxalactam DD and MSA-Cefox [all had sensitivity and specificity of 100% and 86.7% respectively] while among CoNS isolates, diagnosis of MR was better performed by E-test and MSA-Cefox [both had 100% sensitivity and specificity]. We conclude that E-test could be an accurate method in detection of MR. In laboratories that depend mainly on disk diffusion test, a combination of cefoxitin and moxalactam disks should be used to increase the accuracy of the test. MSA-Cefox [4[micro]g/ml] is a promising screening medium with high accuracy. Identification of MR by applying the MRSA screen LAT is a reliable alternative for clinical laboratories where molecular biology techniques are not readily available