Evaluation of different techniques for detection of mecA-mediated resistance in staphylococcus aureus

ABSTRACT

Cefoxitin is a stronger inducer of mecA expression than oxacillin. It is being recommended by Clinical and Laboratory Standards Institute [CLSI] for detection of methicillin resistance in Staphylococcus aureus [MRSA] when using disk diffusion testing. This study was carried out to evaluate the performances of two methods for detection of mecA-mediated resistance in S. aureus strums the automated Vitak 2 system [bi0Me'rieu.x] and the disk diffusion tests, using cefoxitin and oxacillin. Three hundred sixty strains of S. aureus isolated from different clinical samples were used in the study. Oxacillin susceptibility was tested by the Vitak 2 system with the AST-P580 card, [which includes oxacillin MIC and cefoxitin screen] and by disk diffusion methods using l-pg oxacillin disk and 30-pg cefoxitin disc. Oxacillin resistance was confirmed by PCR for the mecA gene "the gold standard method". The 360 clinical isolates of S. aureus studied were divided into 172 MRSA [mecA-positive] and 188 MSSA [mecA negative] isolates. The sensitivities and specificities for the methods evaluated were as follow oxacillin disk diffusion, 97.7 and 95.2%, cefoxitin disk diffusion, 99.4 and 99.5%, Vitak 2 based oxacillin MIC, 98.8 and 95.2%, Vitak 2 cefoxitin screen, 100 and 99.5% and Vitak 2 Advanced Expert System[AES], 100 and 95.2%, respectively. The median times for methicillin testing were 8h for the Vitak 2 system versus 24h for disk diffusion methoa5.h conclusion all of the methods evaluated in the present study appeared to perform very well for the defection of 11dRSA. The results of cefoxitin were in concordance with the PCR for mecA gene. Cefoxitin test either cefoxitin disk diffusion or Vitak 2 cefoxitin screen, were the best performing tests for routine detection of mecA-mediated resistance in S.aureus and the tests can be used as an alternative to the technically demanding PCR, for detection of MRSA