The BD Phoenix system was compared to the cefoxitin disk diffusion test for detection of methicillin (meticillin) resistance in 1,066 Staphylococcus aureus and 1,121 coagulase-negative staphylococcus (CoNS) clinical isolates. The sensitivity for Phoenix was 100%. The specificities were 99.86% for S. aureus and 88.4% for CoNS.Infections caused by methicillin (meticillin)-resistant Staphylococcus aureus (MRSA) or coagulase-negative staphylococci (CoNS) are an increasing problem worldwide (1). Methicillin resistance is primarily due to the presence of a mecA gene which encodes penicillin binding protein 2a (PBP2a) (4).Detection of the mecA gene or PBP2a is considered the gold standard for detecting mecA-mediated methicillin resistance in staphylococci. Among available phenotypic methods, the Clinical and Laboratory Standards Institute (CLSI) has recently introduced the cefoxitin disk diffusion (DD) test for predicting the presence of mecA in S. aureus and CoNS (5, 6), which is preferred over the oxacillin DD test (21). Automated systems are widely used for species identification and susceptibility testing. The aim of the present study was to evaluate the performance of the BD Phoenix automated system (BD, Sparks, MD) in determining methicillin resistance in comparison to the cefoxitin DD test.The study was performed on 1,066 S. aureus isolates and 1,121 CoNS collected during the 2006-2007 routine clinical laboratory activity at the University Hospital of Perugia, Italy. Strains were isolated from the inpatient population of surgical and medical wards and, to a lesser extent, from the outpatient population. Some strains came from other laboratories, for which we are the reference center. Isolates obtained from consecutive cultures from the same patient were excluded. Identification of the isolates was done by conventional methods (colony pigmentation, hemolysis, coagulase production, and the clumping factor test), the Phoenix system (BD), and, in selected cases, the API Staph system (bioMérieux, Marcy l'Etoile, France). Isolates were tested with the cefoxitin DD test, which was performed using Mueller-Hinton agar plates (bioMérieux) and 30-g cefoxitin disks (bioMérieux) and interpreted according to current CLSI breakpoints (6), and with PMIC/ID gram-positive Phoenix panels (BD), which were prepared from subcultures on Columbia sheep blood agar (BD) after isolation on primary plates, according to the manufacturer's instructions. The agreement between both methods was considered the "consensus result." PB2a expression, as detected by the latex agglutination test (Denka Seiken Co., Niigata, Japan), was used to resolve discrepancies. The test was carried out according to the manufacturer's instructions on uninduced inocula for S. aureus (3,22,23) or inocula induced with a 1-g oxacillin disk for CoNS (13,14,17). Methicillin susceptibility in PBP2a-negative CoNS strains was confirmed by mecA gene testing, performed with a LightCycler instrument (Roche Diagnostics, Indianapolis, IN) as described elsewhere (17). For S. aure...
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