Low Vancomycin MICs and Fecal Densities Reduce the Sensitivity of Screening Methods for Vancomycin Resistance in Enterococci

Abstract

cActive surveillance is part of a multifaceted approach used to prevent the spread of vancomycin-resistant enterococci (VRE). The impact of fecal density, the vancomycin MIC of the isolate, and the vancomycin concentration in liquid medium on test performance are uncertain. Using fecal specimens spiked with a collection of 18 VRE (predominantly vanB) with a wide vancomycin MIC range, we compared the performances of commercial chromogenic agars (CHROMagar VRE, chromID VRE, Brilliance VRE, and VRE Select) and 1 liquid medium (Enterococcosel enrichment broth) for VRE detection. The specificity of solid media was excellent; however, the sensitivity at 48 h varied from 78 to 94%. Screening using liquid medium was less sensitive than screening with solid media, particularly as the vancomycin content increased. Sensitivity declined (i) as the fecal VRE density decreased, (ii) when the media were assessed at 24 h (versus 48 h), and (iii) for isolates with a low vancomycin MIC (sensitivity, 25 to 75% versus 100% for isolates with vancomycin MIC of <16 mg/liter versus >32 mg/liter on solid medium using 10 6 CFU/ml of feces). Depending on local epidemiology and in particular VRE vancomycin MICs, the sensitivity of culture-based methods for VRE screening of stool or rectal specimens may be suboptimal, potentially facilitating secondary transmission. Active surveillance is part of a multifaceted infection control approach used to prevent the spread of vancomycin-resistant enterococci (VRE) (1, 2). The ability to detect VRE-colonized patients allows prompt implementation of infection control measures to interrupt the transmission cycle, whereas exclusion of VRE colonization reduces the impact of such activity on patient care and hospital workflow.A variety of in-house and commercial chromogenic solid and liquid media are available for VRE screening in stool or rectal swab specimens. Test performance depends upon a number of variables that may relate to the patient, the specimen, the assay, or the isolate. Previous studies have suggested that the vancomycin MIC of VRE is a determinant of test sensitivity and that the optimal screening method is hence likely to be dependent upon local VRE epidemiology (3, 4). Historically, Australian VRE epidemiology has differed from that in either North America or Europe (5-9), as it is dominated by vanB Enterococcus faecium, of which certain clones have low vancomycin MICs, creating unique challenges for detection during active surveillance (4,10,11 MATERIALS AND METHODSA VRE was defined as an enterococcal isolate that possessed either the vanA or vanB gene regardless of the vancomycin MIC and its relationship to susceptibility breakpoints. This definition has practical validity, as from an infection control perspective, the ability for the resistance mechanism to disseminate is dependent upon the presence of the gene, not the resistance phenotype. Additionally, the vanA and vanB genes are inducible, and hence, MIC expression may be variable (4).Eighteen well-characterized enterococcal stra...

Controlling the spread of vancomycin-resistant enterococci. Is active screening worthwhile?

“…47 When faecal specimens were spiked with 18 VRE isolates, predominantly vanB, sensitivity was lower in liquid compared with solid media, at 24 h compared to 48 h, and when the minimum inhibitory concentration was <16 mg/L. 48 Two main approaches are available via polymerase chain reaction (PCR) systems for the detection of VRE directly from patient specimens, i.e. the LightCycle and the BD Gene Ohm systems.…”

Development of selective medium for IMP-type carbapenemase-producing Enterobacteriaceae in stool specimens

“…Even highly carbapenem-resistant phenotypes such as NDM-type CPE are difficult to identify if cultured as a small inoculum in selective medium [8]. A similar phenomenon is observed when screening for vancomycin-resistant Enterococcus [9]. The sensitivity of screening largely depends on the minimum inhibitory concentration (MIC) of the drug for the isolate and the bacterial load.…”

Comparison of Three Chromogenic Media for Recovery of Vancomycin-Resistant Enterococci from Rectal Swab Samples

“…The other possible reason for the lower sensitivity in this study is that some of the VRE isolates might had a VanD phenotype (vancomycin MICs of 12 to 24 μg/mL, teicoplanin MICs of 4 to 8 μg/mL) despite of their vanA genotype as reported previously [4]. Because both the EB and chromogenic media showed low sensitivity for VRE with low MICs [10].…”