Introduction

Enterococci are Gram-positive cocci that have traditionally been considered to possess low pathogenicity, but have recently emerged as an increasingly common cause of nosocomial infection.  Enterococcus faecalis is the predominant commensal species in humans and causes approximately 90% of clinical infections, whilst Enterococcus faecium is being isolated with increasing frequency.  Most enterococcal infections are endogenous, however cross-infection between hospitalised patients does occur. Vancomycin-resistant enterococci (VRE) were first isolated in Europe in 1986.  The evolution and colonisation of VRE in the European community has been associated with the use of the glycopeptide avoparcin as a growth promoter in pigs and poultry.

Emergence of VRE

Vancomycin resistance in VRE results from one of six known resistance phenotypes.

The resistance phenotypes encode a group of genes that enable synthesis of cell-wall precursors with a terminus modified from the usual D-alanyl-D-alanine group:

• VanA, VanB and VanD produce D-alanyl-D-lactate
• VanC and VanE produce D-alanyl-D-serine.

Glycopeptides such as vancomycin are unable to bind to the altered cell-wall precursor and therefore are unable to interrupt cell-wall synthesis.  VanA and VanB resistance are most commonly seen in Enterococcus faecium and Enterococcus faecalis.

Factors affecting susceptibility to infection

• Vulnerable hospitalised patients.
• Prior antibiotic therapy (especially with glycopeptides or cephalosporins).
• Prolonged hospital stay.

Management of VRE infection

Enterococci are intrinsically resistant to many classes of antibiotics including semisynthetic penicillinase-resistant penicillins, cephalosporins, trimethoprim/sulfamethoxazole and clindamycin.

Principles of treatment

• Indwelling catheters should be removed and any abscesses drained.
• Vancomycin-resistant Enterococcus faecalis:
  • Penicillin or ampicillin, with or without an aminoglycoside.
• Vancomycin-resistant Enterococcus faecium:
  • For ampicillin susceptible strains, ampicillin or an ampicillin/sulbactam combination
  • For ampicillin resistant strains, therapy may include chloramphenicol, rifampicin or tetracycline
  • New agents such as quinopristin/dalfopristin or linezolid are available, although resistance among Enterococcus faecium strains has been demonstrated.
•   Multi-drug resistant VRE:
  • No established drug regimen, current therapies involve experimental combinations.