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Vancomycin-resistant Enterococci (VRE) and the Clinical Laboratory

 

What are the types of vancomycin resistance in enterococci?

There are the two types of vancomycin resistance in enterococci. The first type is intrinsic resistance. Isolates of Enterococcus gallinarum and E. casseliflavus/E. flavescens demonstrate an inherent, low-level resistance to vancomycin.

The second type of vancomycin resistance in enterococci is acquired resistance. Enterococci can become resistant to vancomycin by acquisition of genetic information from another organism. Most commonly, this resistance is seen in E. faecium and E. faecalis, but also has been recognized in E. raffinosus, E. avium, E. durans, and several other enterococcal species.

Several genes, including vanA, vanB, vanC, vanD, and vanE, contribute to resistance to vancomycin in enterococci.

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What are typical vancomycin MICs (phenotypes) for various species of VRE?

E. faecium is the most frequently isolated species of VRE in hospitals and typically produces high vancomycin (>128 µg/ml) and teicoplanin (≥16 µg/ml) minimum inhibitory concentrations (MICs). These isolates typically contain vanA genes. A vanB-containing isolate typically produces lower level resistance to vancomycin (MICs 16 to 64 µg/ml) and is susceptible to teicoplanin (MICs ≤1 µg/ml). Recently, a few vanD-containing isolates of E. faecium with a moderate level of resistance to vancomycin (MICs 64 to 128 µg/ml) and teicoplanin (MICs 4-8 µg/ml) have been reported, as has a novel vanE-containing E. faecalis.

E. gallinarum and E. casseliflavus/E. flavescens isolates are intrinsically resistant to vancomycin. These isolates contain vanC genes that typically produce vancomycin MICs of 2 to 16 µg/ml.

Is identification of VRE to species level important?

Yes. Identification of VRE to species level aids in confirming whether an isolate has intrinsic (vanC) or acquired resistance (vanA or vanB). Knowledge of the type of resistance is critical for infection control purposes. vanA and vanB genes are transferable and can spread from organism to organism. In contrast, vanC genes are not transferable, have been associated less commonly with serious infections, and have not been associated with outbreaks.

For species differentiation, motility and pigment tests are easily performed and are important tests to distinguish among species phenotypically. E. faecium and E. faecalis are non-motile, whereas E. gallinarum and E. casseliflavus/E. flavescens generally are motile. Most isolates of E. casseliflavus/E. flavescens have a distinct yellow pigment, which can be observed by collecting growth from an agar plate on a swab. In addition to motility and pigment tests, an organism's susceptibility profile also helps differentiate vanA and vanB isolates from vanC isolates.

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When should clinical laboratory personnel screen for VRE?

The decision about who and when to screen for VRE is a facility-specific decision. CDC recommendations can assist in the determination of a screening strategy appropriate for healthcare facilities (Recommendations for Preventing the Spread of Vancomycin Resistance Recommendations of the Hospital Infection Control Practices Advisory Committee (HICPAC) MMWR 1995; 44(RR12):1-13). Infection control personnel at some healthcare facilities selectively screen newly admitted or high-risk patients (e.g., intensive care, oncology, and surgery patients) determined to be at greater risk for VRE colonization.

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Why is the difference between colonization and infection important for VRE screening?

Infected patients carry VRE and show clinical signs or symptoms of disease. Colonized patients carry VRE but do not have clinical signs or symptoms of infection. This distinction is important in VRE screening. Patients are usually colonized in the gastrointestinal tract and occasionally in the urinary tract. VRE colony counts are similar in the stool of colonized or infected patients. If a hospital VRE rate is based solely on VRE isolated from clinical cultures (infected patients), the facility may be adequately reporting its infection rate, but may be underestimating the true burden (and therefore potential transmissibility) of VRE in the facility. Screening for patients colonized by VRE provides information about potential sources of illness. The goal of screening is to identify as many colonized patients as possible so that infection control measures can be implemented to decrease transmission and reduce the number of patients infected with VRE.

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How should clinical laboratory personnel screen for VRE?

Screening for VRE can be accomplished in a number of ways. For inoculating peri-rectal/anal swabs or stool specimens directly, one method uses bile esculin azide agar plates containing 6 µg/ml of vancomycin. Black colonies should be identified as an enterococcus to species level and further confirmed as vancomycin resistant by an MIC method before reporting as VRE.

Vancomycin resistance can be determined for enterococcal colonies available in pure culture by inoculating a suspension of the organism onto a commercially available brain heart infusion agar (BHIA) plate containing 6 µg/ml vancomycin. The National Committee for Clinical Laboratory Standards (NCCLS) recommends performing a vancomycin MIC test and also motility and pigment production tests to distinguish species with acquired resistance (vanA and vanB) from those with vanC intrinsic resistance (1).

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Are all VRE of the same species in a hospital clonally related?

Not necessarily. Many molecular sub-types of VRE have been documented, primarily by pulsed-field gel electrophoresis (PFGE). Some hospitals have a large number of VRE that fall into only a few distinct sub-types; one factor causing this could be clonal spread. Other hospitals have isolates of VRE in many sub-types. Many sub-types within a hospital could be caused by different factors, e.g., the hospital may have admitted patients carrying varying molecular sub-types of VRE.

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What methods exist to type VRE to determine clonal spread in a hospital?

Molecular typing of enterococci in outbreak situations is commonly performed by PFGE. Banding patterns produced by each organism are matched, and this information is combined with epidemiologic data to determine relatedness between strains. Other molecular typing systems include PCR-based typing methods, multilocus enzyme electrophoresis, and ribotyping.

(1) National Committee for Clinical Laboratory Standards. 1999. Performance standards for antimicrobial susceptibility testing. NCCLS approved standard M100-S9. National Committee for Clinical Laboratory Standards, Wayne, PA.

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