Convenient Selective Differential Broth for Isolation of Vancomycin-Resistant Enterococcus from Fecal Material

Abstract

Studies have shown that vancomycin broth enrichment is superior to direct plating for the detection of vancomycin-resistant enterococcus (VRE), but vancomycin selective broth is not generally commercially available. We developed an easy-to-prepare VRE selective differential broth and compared it to direct plating on bile esculin azide (BEA) agar for the isolation of VRE from fecal samples. A total of 528 consecutive rectal swabs and stools were inoculated onto BEA agar and into BEA broth with vancomycin at a concentration of 15 g/ml (BEA VAN 15g/ml broth). After 1 to 2 days of incubation, broths were subcultured to BEA VAN 6g/ml agar. Bile esculin-positive colonies from the direct and broth subculture plates were evaluated for the presence of VRE by standard microbiological techniques. Addition of the broth enrichment step led to the detection of significantly more VRE isolates than did direct plating alone (28 versus 18 VRE isolates, respectively). In all, 30 VRE strains were isolated from 29 cultures, all of which were Enterococcus faecium. MICs of vancomycin ranged from 32 g/ml (n ‫؍‬ 2) to > 256 g/ml (n ‫؍‬ 28). Twenty-two VRE isolates were available for further testing: sixteen exhibited a VanA phenotype and six were of the VanB phenotype. van genotypes were in agreement with phenotypes for all VRE isolates except one, which could not be genotyped. The broth method also resulted in significantly fewer bile esculin-positive, non-VRE isolates requiring further workup. We have thus developed an easily prepared vancomycin selective differential broth that is significantly more sensitive and specific in the detection of VRE than is direct fecal plating to BEA agar.

Evaluation of a New Chromogenic Medium, chromID VRE, for Detection of Vancomycin-Resistant Enterococci in Stool Samples and Rectal Swabs

“…The sensitivities of the chromID VRE and BEAv media were similar under various conditions of use (Table 1, P Ͼ 0.05). The rate of VRE detection was significantly increased by the enrichment step, as previously demonstrated (7,11,16). However, fecal carriage of these VRE was transitory in patients and this increased sensitivity may not be essential for the management of most outbreaks (4).…”

Laboratory and Clinical Evaluation of Screening Agar Plates for Detection of Carbapenem-Resistant Enterobacteriaceae from Surveillance Rectal Swabs

“…A summary of these studies is presented in Table 3. The differences in studied populations, laboratory methods, and study design make a head-to-head comparison difficult, but several conclusions can be made: (i) among agar-based methods, the use of carbapenem disks on MacConkey plates appears to be the least sensitive, and (ii) unlike screening for vancomycin-resistant enterococci (18), enrichment in broth was not superior to direct plating (13). Our study adds to the current literature the direct comparison of MacI with two previously described agarbased methods, along with an examination of the analytic sensitivity of these methods in detecting various types of CRE strains.…”

Comparison of PCR and Culture for Screening of Vancomycin-Resistant Enterococci: Highly Disparate Results for vanA and vanB

“…A limitation of this study is that an enrichment broth was not used and may have affected the sensitivity of VRE isolation by culture. While some studies have demonstrated that broth enrichment may increase the VRE detection by 10 to 30% (10,12), the number of specimens positive for vanB by PCR yet negative by culture far exceeds what one would expect even if an enrichment broth was used.…”