Enterococcus

Background

Microbiology

• Genus of facultative anaerobic, non-spore-forming, alpha- or gamma-hemolytic, catalase negative, Gram-positive cocci
• Grows on bile esculin agar
• PYR positive
• Most are Lancefield group D
• Commensal gut flora
• Increasing antibiotic resistance

Species

• Enterococcus faecalis
  • More common (90-95%)
  • More commonly genitourinary source
  • More susceptible to antibiotics
• Enterococcus faecium
  • Less common (5-10%)
  • More commonly gastrointestinal source
  • Less susceptible to antibiotics
  • Most common VRE
• Enterococcus avium
• Enterococcus gallinarum
• Enterococcus casseliflavus
• Enterococcus hirae, a rare zoonotic pathogen from chickens that can cause bacteremia and endocarditis

Vancomycin Resistance

• Vancomycin binds to d-Ala-d-Ala pentapeptids, interfering with cell wall synthesis
• VanA: most common. Mutation to d-Ala-d-Lac, leading to high resistance to glycopeptides.
• VanB: more common in E. faecium in Australia
• VanC: chromosomal resistance on E. gallinarum and E. casseliflavus

Management

• Vancomycin is usually reliable (except for Enterococcus gallinarum and Enterococcus casseliflavus)
• Ampicillin or amoxicillin is preferred for susceptible strains
  • Piperacillin is effective, as well, but not cloxacillin
• Inherent resistance to cephalosporins
• Resistant to ertapenem, but ampicillin-susceptible strains are often susceptible to imipenem and (less reliably) to meropenem12

VRE

• Daptomycin, doxycycline, linezolid, tedizolid, oritavancin, quinupristin-dalfopristin (for E. faecium)
• For VanC isolates, ampicillin or penicillin is preferred, and daptomycin susceptibility is often retained3