Performance of a New MicroScan WalkAway PC30 Panel and Disk Diffusion Method for Detection of Oxacillin Resistance in Staphylococcus spp

Abstract

The performance of the MicroScan WalkAway PC30 panel for detection of oxacillin resistance was evaluated by use of a collection of 420 staphylococcus isolates. The addition of a cefoxitin test (4 mg/liter) to the oxacillin MIC determination increased its raw performance for Staphylococcus aureus; additional data were required for coagulase-negative staphylococci.Detection of oxacillin (OXA) resistance (OR) in staphylococci is a daily challenge for clinical laboratories. Erroneous susceptibility results could lead to therapeutic failure or inadequate antimicrobial selection pressure (5, 18). Cefoxitin (FOX) testing is currently recommended and used for this purpose (2). We evaluated the performance of a new MicroScan WalkAway panel (Siemens, Sacramento, CA) that includes this compound and the disk diffusion method (DDM) for the detection of OR.(This work was presented in part at the 20th European Congress of Clinical Microbiology and Infectious Diseases [ECCMID], Vienna, Austria, 10 to 13 April 2010.)A set of 420 nonduplicate isolates of Staphylococcus aureus (n ϭ 370) and coagulase-negative staphylococci (CNS; n ϭ 50) was collected from two nationwide studies through the French College de Bacteriologie-Virologie-Hygiene network, comprising isolates from cases of community-acquired skin and soft tissue infections (203 S. aureus isolates in 2006) and clinically significant bacteremia (167 S. aureus and 50 CNS isolates in 2007) (9, 10). Identification was performed using (i) specific gyrB PCR for S. aureus, (ii) mass spectrometry (Axima Assurance; Shimadzu) for CNS, and (iii) 16S RNA sequencing when the identification differed from that obtained at the participating-laboratory level (3,4,11). The detection of the mecA gene was performed as previously described (10). OR was detected by (i) the FOX DDM according to CLSI guidelines, (ii) the moxalactam (MOX) DDM using the French official criteria (OR is shown by a diameter of Ͻ23 mm), and (iii) the MicroScan WalkAway PC30 panel (Siemens, Sacramento, CA) (13,14). This panel contained oxacillin in doubling dilutions from 0.25 to 2 g/ml and an additional FOX test (4 g/ml); each strain was categorized as OR if its oxacillin MIC was greater than 2 g/ml for S. aureus and 0.5 g/ml for CNS or if the FOX test was positive (i.e., yielded a growth). The panels were run automatically according to the manufacturer's instructions. Quality control strains (oxacillin-susceptible S. aureus ATCC 25923 and OR S. aureus [ORSA] ATCC 43300) were included in each sample set. All results were within the acceptable range provided by the 2008 CLSI guidelines (2). The contribution of the FOX test was statistically evaluated with the Fisher's exact test. A P value of Յ0.05 was considered to reflect significance. All computations were done with R Project software (http://www.r-project.org).OR was detected by PCR in 22.7% of S. aureus (84/370) and 66% of CNS (33/50) isolates; the latter group included isolates of S. epidermidis (34/50), S. haemolyticus (8/50), S. hominis (5/50), S. capitis (2/50),...

Evolution of Nasal Carriage of Methicillin-Resistant Coagulase-Negative Staphylococci in a Remote Population

“…(28,29). Precise characteristics of this study population have been previously described (12,37,50 (37) were defrosted in a batch, and 100 l of the suspension was plated on chromID MRSA agar (bioMérieux), a chromogenic agar selective for MR staphylococci (cefoxitin, 4 g/liter) with a Ͼ90% sensitivity and a 94 to 100% specificity to discriminate mecA-positive and mecA-negative CoNS isolates, including for the Staphylococcus saprophyticus species (10,35,36). After incubation for 48 h at 37°C, four white colonies were randomly selected from each plate and subcultured on Trypticase soy agar.…”