Detection of Methicillin-Resistance of Coagulase-negative Staphylococci

Cho, Young Uk; Chae, Jeong Don; Park, Hye Young; Kim, Mi Na

Abstract

BACKGROUND: Coagulase-negative staphylococci (CNS) has been considered as a major causative agent of nosocomial infections. A prompt and accurate detection of methicillin resistance (MR) in staphylococci is a current issue of clinical microbiology laboratories. This study was purposed to evaluate various methods for detecting MR from CNS.
METHODS
We selected 78 CNS strains obtained from blood cultures from April 1999 through July 2001 including 20 strains of Staphylococcus epidermidis, 20 S. hominis (SHO), 19 S. capitis, 9 S. haemolyticus, 3 S. saccharolyticus, 1 S. saprophyticus (SAP), 2 S. warneri (SWA), 2 S. xylosus, 1 S. lugdunensis, and 1 S. auricularis. In addition, one SAP strain received from World Health Organization for proficiency tests was also studied. The following methods were compared to the mecA gene PCR: MicroScan PosCombo 12, oxacillin salt agar containing 6 microgram/mL (OSA-6) or 0.6 microgram/mL (OSA- 0.6) of oxacillin, oxacillin disk diffusion (ODD), and MRSA-Screen latex agglutination (LA) for detecting penicillin binding protein 2a.
RESULTS
One SWA was failed in mecA-PCR and fifty-nine of 78 (75.6%) CNS were positive for mecA gene. The agreement rates, sensitivities, and specificities for each test were as follows: for MicroScan, 97.3%, 98.2%, 88.9%; for OSA-6 and OSA-0.6 at 24-h incubation, 79.5%, 74.6%, 94.7% and 79.5%, 72.9%, 100%, respectively, and at 48-h incubation, 91.0%, 91.5%, 89.5% and 91.0%, 96.6%, 73.7%, respectively; ODD, 84.6%, 84.7%, 84.2%; LA, 80.8%, 76.3%, 94.7%. One SHO and one SAP that were mecA-negative showed resistance in the MicroScan, ODD, and OSA.
CONCLUSIONS
MicroScan appears a reliable method to detect MR in all species of CNS except SHO and SAP. ODD and LA were not appropriate in detecting MRCNS due to a low sensitivity. Although OSA-0.6 at 48-h incubation showed a high sensitivity, the low specificity may limit a routine use in clinical laboratory.