Correlation of Cefoxitin MICs with the Presence of mecA in Staphylococcus spp

Abstract

This report describes the results of an 11-laboratory study to determine if a cefoxitin broth microdilution MIC test could predict the presence of mecA in staphylococci. Using breakpoints of <4 g/ml for mecA-negative and >6 or 8 g/ml for mecA-positive isolates, sensitivity and specificity based on mecA or presumed mecA for Staphylococcus aureus at 18 h of incubation were 99.7 to 100% in three cation-adjusted Mueller-Hinton broths tested. For coagulase-negative strains at 24 h of incubation, breakpoints of <2 g/ml for mecA-negative and >4 g/ml for mecA-positive isolates gave sensitivity and specificity of 94 to 99% and 69 to 80%, respectively.The use of a cefoxitin disk test to detect staphylococci that are likely to contain the mecA gene has been widely advocated since the test was first suggested (3, 6) and has been adopted by antimicrobial susceptibility testing organizations worldwide (http://www.bsac.org.uk/_db/_documents/version_6.1.pdf, http: //www.srga.org/, http://www.clsi.org/). For laboratories that do not use disk diffusion as their primary testing method, performing disk diffusion requires additional expense and reagents. Consequently, we performed studies to determine if a cefoxitin broth microdilution MIC breakpoint would correlate with the presence of the mecA gene in staphylococci.Cation-adjusted Mueller-Hinton broths (CAMHB) from three manufacturers (BBL, BD Diagnostic Systems, Sparks, MD; Difco, BD Diagnostic Systems, Sparks, MD; and Oxoid, Basingstoke, Hampshire, England) (cation content adjusted if necessary), were used to prepare frozen microdilution panels containing cefoxitin concentrations of 0.5 to 32 g/ml (including 6 g/ml) and oxacillin concentrations of 0.06 to 16 g/ml (in BBL CAMHB only). The frozen panels were shipped to all participants, along with 30-g cefoxitin disks (BBL). Each laboratory used its current lot of Mueller-Hinton agar for disk diffusion testing. ). Additional strains were tested at the CDC, totaling 167 S. aureus isolates and 22 CoNS. The total number of strains tested in all labs was 479 S. aureus isolates and 204 CoNS. All results were read after both 18-h and 24-h incubations. Endpoints were read as the lowest concentration at which no visible growth was observed. At 18 h of incubation, 97.3% of the MIC quality control results (for S. aureus ATCC 29213) and 97.9% of the disk diffusion quality control results (for S. aureus ATCC 25923) were within the range considered acceptable by the Clinical and Laboratory Standards Institute (CLSI).After testing, all strains were shipped to the CDC for further analysis. PCR testing for mecA was performed as previously described (8) on the following strains: (i) any S. aureus strain for which the cefoxitin MIC was Ͻ16 g/ml in any of the three media tested (n ϭ 180); (ii) any S. aureus strain for which the cefoxitin MIC was Ͼ8 g/ml and the oxacillin MIC was Ͻ4 g/ml in any of the three media tested (n ϭ 5); and (iii) a sample of the 304 isolates (n ϭ 16 [5%]) for which the cefoxitin MIC was Ͼ8 g/ml and the oxacillin MIC was Ͼ2 g...

Detection of Methicillin-Resistant Coagulase-Negative Staphylococci by the Vitek 2 System

“…However, detection of MR CoNS is problematic and less reliable than the detection of MR Staphylococcus aureus (1,2). Cefoxitin susceptibility testing has greatly improved the reliability of detecting MR S. aureus and, to a lesser extent, CoNS (3,4). Current CLSI guidelines recommend the use of cefoxitin disk testing for detecting MR CoNS, with some evidence that cefoxitin MIC determination can serve the same purpose (3,5).…”

“…Cefoxitin susceptibility testing has greatly improved the reliability of detecting MR S. aureus and, to a lesser extent, CoNS (3,4). Current CLSI guidelines recommend the use of cefoxitin disk testing for detecting MR CoNS, with some evidence that cefoxitin MIC determination can serve the same purpose (3,5). A number of studies have shown that cefoxitin disk testing of CoNS is generally sensitive but can be nonspecific (3,4,6,7).…”

“…Current CLSI guidelines recommend the use of cefoxitin disk testing for detecting MR CoNS, with some evidence that cefoxitin MIC determination can serve the same purpose (3,5). A number of studies have shown that cefoxitin disk testing of CoNS is generally sensitive but can be nonspecific (3,4,6,7). The Vitek 2 (Vitek) antimicrobial susceptibility system utilizes a cefoxitin susceptibility screening assay that was previously reported to have a 98% sensitivity for MR and a 100% specificity for S. epidermidis but only 66% and 100% sensitivity and specificity, respectively, for S. hominis (6).…”

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

“…The ability of the FOX test to detect this kind of low-level OR should be confirmed in a larger study. Concerning the CNS strains, the contribution of the FOX tests (DDM and panel PC30) was poor, as previously reported (21). However, the number of strains was insufficient to draw any conclusion, the limited power rendering irrelevant the meaning of the not-significant P value.…”

“…Recently, a MIC value of 4 g/ml was established for FOX to detect the presence of the mecA gene (21). In the study of Swenson et al, only 0.3% (1/312) of mecA-positive S. aureus isolates showed a FOX MIC inferior to this breakpoint (21).…”