Enterococcus species

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

• E. faecalis
  • More common (90-95%)
  • More commonly genitourinary source
  • More susceptible to antibiotics
• E. faecium
  • Less common (5-10%)
  • More commonly gastrointestinal source
  • Less susceptible to antibiotics
  • Most common VRE
• E. gallinarum
• E. casseliflavus
• E. 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 E. gallinarum and E. casseliflavus)
• Ampicillin is preferred for susceptible strains
• Inherent resistance to cephalosporins
• Resistant to ertapenem, but ampicillin-susceptible strains are often susceptible to imipenem and (somewhat) meropenem

VRE

• Daptomycin, doxycycline, linezolid, tedizolid, oritavancin, quinupristin-dalfopristin (for E. faecium)