Comparison of simple and rapid methods for identifying enterococci intrinsically resistant to vancomycin

Evaluation of a PCR assay to detect enterococcusfaecalis in blood and determine glycopeptides resistance genes: van a and van B

BACKGROUND

Bacteremia due to Enterococcus faecalis is usually caused by strains resistant to most antibiotics. Effective management of the disease is dependent on rapid detection and characterization of the bacteria, and determination its sensitivity pattern to antimicrobial drugs. The aim of this study was to investigate a more rapid and reliable assay for simultaneous diagnosis of enterococcal bacteremia and its sensitivity pattern to antimicrobial drugs.

METHODS

Several bacterial suspensions with different content of two standard strains of Enterococcus faecalis resistant to vancomycin were used for inoculation to defibrinated sheep blood samples. PCR and routine assay was performed on all blood samples with different bacterial content.

RESULTS

Routine assay and PCR for all inoculated blood samples with ≥5 cfu/ml was positive. Mean time for PCR and routine assays was 10 hours and 5 days, respectively.

CONCLUSION

PCR is a more rapid and sensitive assay for simultaneous detection and characterization for Enterococcus faecalis, and determination of its sensitivity pattern to vancomycin.

Outbreak of vancomycin resistant enterococcus in a hematology/oncology unit in a Korean university hospital, and risk factors related to patients, staff, hospital care and facilities

A vancomycin-resistant Enterococcus (VRE) outbreak occurred at a hematology/oncology (HO) service in a tertiary acute care hospital, 1 January to 30 April 2002. This study was undertaken to identify risk factors, and to establish relations between seasonal factors and this VRE outbreak. Every patient (n = 82) and health care worker (HCW) (n = 31) at the HO unit was screened by rectal swab and stool culture during the VRE outbreak. Environmental surface culture (n = 35) and pulsed-field gel electrophoresis (PFGE) of VRE isolates were preformed. Among a total of 82 patients, 17 had VRE. All VRE isolates were E. faecium, the 6 studied further were Van A with identical PFGE patterns. Cultures of surfaces and HCWs were negative for VRE. Prolonged hospital stay (p = 0.02), male gender (p = 0.02), and care in a 6-bed room (p < 0.01) were risk factors for acquisition of VRE. An association with the VRE outbreak period was found again for long hospital stay, but also for the antibiotic usage pattern, frequent surgery and a high proportion of new inexperienced staff (52% vs 4% before the outbreak (p < 0.01, Fisher's exact test). Sudden personnel turnover in high-risk units should be avoided.

Evaluation of 15 motility media and a direct microscopic method for detection of motility in enterococci

Isolation of motile, vanC enterococci has yet to be a major infection control concern; however, rapid detection still is important. We evaluated 15 motility media from three manufacturers and a 2-h direct microscopic method for accurate detection of 89 enterococcal strains, including 72 vanC enterococcal strains. Resistance genes were confirmed by a multiplex PCR method with the vanC gene detected in all motile enterococci. Motility in the 72 vanC enterococci was detected at 30 degrees C within 72 h in BD Biosciences motility nitrate medium and in Remel motility B medium, motility B medium supplemented with methyl-alpha-D-glucopyranoside (investigational), motility S medium, motility test medium, and motility test medium with tetrazolium indicator. Motility was also observed for all vanC enterococci with the 2-h direct (30 degrees C incubation) microscopic detection method. All Enterococcus faecalis and Enterococcus faecium isolates were observed to be nonmotile in all media and by the direct microscopic method. Since differences between the various motility media tested were observed, the medium used for detection of enterococcal motility must be selected carefully.

Routine molecular identification of enterococci by gene-specific PCR and 16S ribosomal DNA sequencing

For 279 clinically isolated specimens identified by commercial kits as enterococci, genotypic identification was performed by two multiplex PCRs, one with ddl(E. faecalis) and ddl(E. faecium) primers and another with vanC-1 and vanC-2/3 primers, and by 16S ribosomal DNA (rDNA) sequencing. For 253 strains, phenotypic and genotypic results were the same. Multiplex PCR allowed for the identification of 13 discordant results. Six strains were not enterococci and were identified by 16S rDNA sequencing. For 5 discordant and 10 concordant enterococcal strains, 16S rDNA sequencing was needed. Because many supplementary tests are frequently necessary for phenotypic identification, the molecular approach is a good alternative.

Evaluation of D-xylose and 1% methyl-alpha-D-glucopyranoside fermentation tests for distinguishing Enterococcus gallinarum from Enterococcus faecium

To determine the validity of the rapid xylose and methyl-alpha-D-glucopyranoside (MDG) fermentation tests in distinguishing Enterococcus gallinarum from Enterococcus faecium, 156 well-characterized clinical isolates of enterococci (55 E. gallinarum, 91 E. faecium, and 10 Enterococcus faecalis isolates) known to be of different clones were examined in a blinded fashion. Species identification was confirmed by PCR of the ddl ligase genes of E. faecium and E. faecalis and the vanC1 gene of E. gallinarum. Xylose tests were performed with D-xylose tablets by using a heavy bacterial suspension and were interpreted after 2 h of incubation. Standard MDG fermentation tests were read after 24 h of incubation. The xylose fermentation test had a sensitivity of 98% (54 of 55) and a specificity of 99% (100 of 101) in distinguishing E. gallinarum from E. faecium and E. faecalis. The standard MDG test had a sensitivity of 100% (55 of 55) and a specificity of 95% (96 of 101) after 24 h. The xylose fermentation test is a simple method, easily incorporated into laboratory protocols, that distinguishes E. gallinarum from E. faecium with high sensitivity and specificity in 2 h. The standard MDG test has high sensitivity and can be useful in ruling out the presence of E. gallinarum but requires overnight incubation.

Comparison of phenotypic methods to identify enterococci intrinsically resistant to vancomycin (VanC VRE)

Clinical laboratories must be able to differentiate between enterococci possessing acquired resistance to vancomycin (vanA and vanB genotypes) from those that are inherently resistant (vanC1 and vanC2/C3 genotypes). We compared several routine phenotypic tests to determine the species identity of clinical isolates of enterococci and a PCR assay for the van ligase genes was used to confirm identification of VanC VRE. The Vitek Gram Positive Identification card identified 53/60 (88%) Enterococcus faecalis and E. faecium isolates and 81/141 (57%) VanC VRE without additional testing. Another 32 of the VanC VRE required additional testing (e.g., motility and pigmentation) for correct identification. However, 7 of these 32 VanC VRE were nonmotile. The rapid ID 32 STREP strips identified 50/60 (83%) E. faecalis and E. faecium isolates and 102/141 (72%) VanC VRE. All E. faecalis and E. faecium isolates were nonmotile and did not acidify 1% methyl-alpha-D-glucopyranoside (MGP). Only 93/115 (81%) E. gallinarum and 21/26 (81%) E. casseliflavus/E. flavescens were motile but all 141 VanC VRE acidified MGP. MGP acidification can accurately differentiate VanC VRE from E. faecalis and E. faecium. Because some VanC VRE isolates are nonmotile, MGP acidification is preferred as a simple and less costly test for identification of these isolates.

Evaluation of the revised MicroScan dried overnight gram-positive identification panel to identify Enterococcus species

The revised MicroScan Dried Overnight Gram-Positive Identification panel was evaluated for its efficacy at identifying Enterococcus species in comparison with conventional biochemical tests. Supplemental testing of ampicillin-susceptible Enterococcus faecium for motility and the ability to acidify methyl-alpha-D-glucopyranoside helped recognize E. gallinarum and increased the accuracy of the panel for identifying Enterococcus species to 98.5%.

An Update on the Emergence of Glycopeptide Resistance in Enterococci

Glycopeptide resistance may be either constitutive or transferable (on plasmids or as a transposon), and four phenotypes (van A, B, C, D) have been described to date. Recent data suggest solid media screening protocols appear to be insensitive at detecting low levels of carriage, and up to 40% of colonized patients may be falsely glycopeptide-resistant enterococci (GRE) negative. Managing GRE-colonized or -infected patients using contact precautions appears to be useful in controlling clonal outbreaks, but may be of limited utility once GRE is endemic. Alternate strategies to manage GRE-colonized patients with prolonged carriage and in outpatient or home health settings include using risk-based transmission assessment to limit the logistic and psychosocial difficulties associated with the use of continuous contact precautions. The therapeutic options for treating GRE infection remain limited. Attempts to decolonize GRE-colonized patients with bacitracin appear to be of limited utility.