Evaluation of phenotypic and genotypic methods for detection of methicillin resistance in Staphylococcus aureus

Abstract

Rapid and reliable identification of methicillin-resistant Staphylococcus aureus(MRSA) is important for provision of both appropriate therapy and control measures. However, the heterogeneous nature of methicillin resistance in S. aureus limits accuracy and reliability of phenotypic methods for detecting resistance. In this study, phenotypic methods for determination of methicillin resistance were compared with polymerase chain reaction (PCR) for detection of mecA gene. A total of 160 clinical isolates of S. aureus were tested to detect resistance by disc diffusion method, MRSA screen test, and MiniAPI System. The minimum inhibitory concentration values of oxacillin were also determined by broth microdilution method in 115 isolates. A total of 87 (54.4%) isolates were mecA positive and 73 (45.6%) isolates were mecA negative. The sensitivities of disc diffusion method, broth microdilution method, MRSA screen test, and MiniAPI System were 98.8, 98.2, 97.7 and 98.8%; and specificities were 97.2, 98.2, 94.5 and 89% respectively considering PCR as the reference method. The differences in sensitivities or specificities were not statistically significant (p > 0.05). We conclude that an algorithm should be designated for correct identification of MRSA in routine laboratories because none of the phenotypic tests is completely reliable for the detection of methicillin resistance in S. aureus. Key words: Methicillin resistance, Staphylococcus aureus, polymerase chain reaction