Evaluation of phenotypic with genotypic methods for species identification and detection of methicillin resistant in Staphylococcus aureus.

BACKGROUND

Phenotypic methods for the detection of methicillin resistance are inadequate, due to presence of hetero-resistant population and dependence of environmental factors that may affect the phenotypic expression of resistance.

AIMS

Present study was conducted, to evaluate the efficacy of phenotypic methods for the identification of species and mec-A mediated resistance in S. aureus with polymerase chain reaction (PCR), and to assess the prevalence of the Panton-Valentine leukocidin (pvl) toxin in methicillin resistant S. aureus (MRSA) and overall S.aureus population.

MATERIALS AND METHODS

A total of 200 clinical isolates of Staphylococci were subjected to phenotypic and genotypic methods for the species identification and detection of MRSA.

RESULTS

The specificity and sensitivity of conventional methods in the detection of S.aureus, was found to be 100 and 97.59% respectively. However, the performance of phenotypic methods in the detection of MRSA were: Oxacillin disc diffusion (DD)-sensitivity 70.58%, specificity 75.75%; cefoxitin DD-sensitivity 86.27%, specificity 83.33%; and oxacillin agar dilution-sensitivity 92.15%, specificity 90.90%. PVL gene was detected in all mec-A positive isolates irrespective of their types.

CONCLUSION

Phenotypic methods still preferred for the species identification, but for the reliable detection of MRSA an algorithm should include a combination of tests and apply a genotypic method for confirmation of resistance isolates showing discrepant results. Considering the high prevalence of PVL-MRSA, we recommend PCR as assay, as it has an advantage of simultaneous detection of mec-A and pvl genes by multiplex PCR.