[Accuracy of four agar diffusion methods and the Vitek 2 automated system for the detection of the methicillin resistance in coagulase negative staphylococci].
OBJECTIVE
Technical difficulties may occur in the detection of methicillin resistance in coagulase negative staphylococci. Phenotypic methods, such as disk diffusion testing, sometimes fail to detect methicillin resistance because of its poor expression. Four of these methods were compared with the VITEK 2 system.


METHODS
The accuracy of the VITEK 2 system and the agar diffusion methods according to the current CASFM and NCCLS guidelines were assessed for methicillin susceptibility testing of 70 coagulase negative staphylococci isolates harboring the mecA gene for which the detection of the methicillin resistance was particularly difficult: the CASFM recommendations applied in 1998 failed to categorize them as methicillin-resistant.


RESULTS
Among the different methods currently suggested by the CASFM, those using a heavy inoculum and a hypersaline medium give better results, but they remain however less efficient than the NCCLS recommendations testing (sensitivity: 71-86% vs. 94-94% respectively). The VITEK 2 was more efficient (sensitivity: 91%) than the disk diffusion methods recommended by the CASFM and slightly less efficient than the method recommended by the NCCLS.


CONCLUSIONS
The VITEK 2 automated system, compared with the disk diffusion methods, showed adequate accuracy for detection of oxacillin resistance in coagulase negative staphylococci.
Aïssa
N
N
Service de bactériologie-virologie-hygiène, hôpital Henri-Mondor, 51, avenue du Maréchal-de-Lattre-de-Tassigny, 94010 Créteil cedex, France.
Stolar
D
D
Legrand
P
P
D003030:Coagulase / Q000172:deficiency; D015169:Colony Count, Microbial / Q000379:methods*; D008712:Methicillin / Q000494:pharmacology; D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013210:Staphylococcus / Q000187:drug effects*
D003030:Coagulase; D008712:Methicillin