[Comparison of phenotypic methods with PCR to screening methicillin resistance in coagulase negative staphylococci]

Appreciation of the frequency, the level and the genetic support of methicillin resistance.

Seventy-three strains of coagulase negative staphylococci isolated from various specimens, from January to June 2004, were studied. The phenotypic detection was carried out by disk diffusion test using oxacillin and cefoxitin disks, by the determination of oxacillin Minimal Inhibitor Concentration (E-test), by the oxacillin screening test at a concentration of 4 mug/ml and by the search of the penicillin binding protein PBP2a using the slide latex agglutination test. The results of these methods were compared to PCR of mecA gene.

Forty-eight strains carried mecA gene whose 30 were detected by the oxacillin disk, the cefoxitin disk, the oxacillin screening test, the slide latex agglutination test and had a MIC from 24 to 256 mug/ml. Seventeen strains were not detected by oxacillin disk but by cefoxitin disk and the slide latex agglutination test. Among these strains, 13 (76%) had oxacillin MIC from 0.5 to 1,5 mug/ml and not grew on oxacillin agar screening, while 4 (24%) had oxacillin MIC from 6 to 16 mug/ml and grew on this agar. One strain had oxacillin MIC of 0,19 mug/ml and was not detected with any phenotypic method.

The determination of oxacillin MIC, the search of the PBP2a or more simply the cefoxitin disk had permitted to detect the strains mecA gene (+) with resistant and pre-resistant phenotype but not the strain with sensible phenotype (2.1%).