Vancomycin-Resistant Enterococcus

Vancomycin-resistant enterococci (VRE) are strains of Enterococcus, chiefly Enterococcus faecium, that have acquired resistance to vancomycin, a glycopeptide antibiotic central to treating Gram-positive infections. Because enterococci are already intrinsically tolerant of several antibiotic classes, the additional loss of glycopeptide activity makes VRE an important multidrug-resistant cause of healthcare-associated infection.

Definition

Vancomycin-resistant enterococci are Enterococcus strains that have acquired gene clusters (most often vanA or vanB) which remodel the cell-wall peptidoglycan precursor so that vancomycin can no longer bind effectively, producing resistance to the glycopeptide.

Scope

The entry covers the genetic basis of glycopeptide resistance in enterococci, the cell-wall target alteration that confers it, the principal resistance gene clusters, and the epidemiologic significance of VRE. It is descriptive reference material and does not provide treatment selection or dosing guidance.

Core questions

• What change in the bacterial cell wall confers vancomycin resistance in enterococci?
• How do the vanA and vanB resistance phenotypes differ?
• Why are enterococci predisposed to becoming clinically important multidrug-resistant pathogens?

Key concepts

• Glycopeptide resistance
• vanA and vanB gene clusters
• D-alanyl-D-alanine to D-alanyl-D-lactate target change
• Enterococcus faecium
• Intrinsic resistance of enterococci
• Gut colonization and healthcare transmission
• Mobile genetic elements

Mechanisms

Vancomycin normally binds the terminal D-alanyl-D-alanine of the peptidoglycan precursor, blocking cell-wall cross-linking. In VRE, acquired van gene clusters direct synthesis of an altered precursor ending in D-alanyl-D-lactate (or D-alanyl-D-serine), to which vancomycin binds with greatly reduced affinity, so cell-wall synthesis proceeds despite the drug. The most common clusters are vanA, typically conferring high-level resistance to vancomycin and teicoplanin and often carried on transposons, and vanB, which confers variable vancomycin resistance. Because these determinants reside on mobile genetic elements, they can transfer between enterococci.

Clinical relevance

VRE cause bloodstream infections, urinary and intra-abdominal infections, and device-related infection, predominantly in hospitalized, immunocompromised, or heavily antibiotic-exposed patients, and the loss of glycopeptide activity on top of intrinsic enterococcal resistance narrows options, making VRE a target of infection control and stewardship. This entry describes the resistance mechanism for educational reference and is not a basis for selecting therapy in an individual case.

Epidemiology

Vancomycin resistance is concentrated in Enterococcus faecium, which spreads in healthcare settings through gut colonization and environmental contamination, particularly among patients with prolonged hospital stays and broad antibiotic exposure. VRE, especially vancomycin-resistant Enterococcus faecium, appears on international priority pathogen lists.

History

Vancomycin-resistant enterococci were first reported in the late 1980s and subsequently spread in hospitals worldwide, with Enterococcus faecium becoming the predominant resistant species; the dissemination of vanA and vanB clusters on mobile elements established VRE as a persistent healthcare-associated concern.

Debates

What is the clinical significance of enterococcal colonization versus infection?

Enterococci frequently colonize the gut without causing disease, so distinguishing colonization from true infection, and deciding when VRE colonization warrants infection-control action, remains a practical question.

Seminal works

• deoliveira-2020
• tacconelli-2018

Frequently asked questions

How does vancomycin resistance actually work in enterococci?

Acquired van genes change the end of the cell-wall building block from D-alanyl-D-alanine to D-alanyl-D-lactate, which vancomycin binds much more weakly, so the drug can no longer block cell-wall synthesis.

Which Enterococcus species is most often vancomycin-resistant?

Enterococcus faecium accounts for most clinically important vancomycin-resistant enterococci, although resistance can also occur in Enterococcus faecalis.