What is the recommended dosage of amoxicillin (amoxicillin) for treating Enterococcus faecalis (E. faecalis) infections?

Amoxicillin Dosage for Enterococcus faecalis Infections

For uncomplicated E. faecalis infections, amoxicillin at a dosage of 40-90 mg/kg/day divided in 2-3 doses for 5-7 days is recommended, while serious infections such as endocarditis require higher doses with combination therapy. 1

Treatment Approach Based on Infection Severity

Uncomplicated Infections

• For uncomplicated E. faecalis infections (e.g., simple UTIs):
  • Amoxicillin 40-90 mg/kg/day divided in 2-3 doses for 5-7 days 1
  • Alternative for uncomplicated UTIs: Fosfomycin 3g single dose or nitrofurantoin 100mg twice daily for 5 days 1

Serious Infections (Endocarditis, Bacteremia)

For serious E. faecalis infections, combination therapy is typically required:

1. First-line regimen (penicillin-susceptible strains):

   • Ampicillin or amoxicillin plus gentamicin:
     • Amoxicillin/ampicillin: 2g IV every 4-6 hours 1
     • Gentamicin: 3 mg/kg/day IV divided in 2-3 doses 2, 1
   • Duration: 4-6 weeks for endocarditis 1

2. Alternative regimen (high-level aminoglycoside resistance):

   • Ampicillin/amoxicillin plus ceftriaxone 1
   • This combination avoids nephrotoxicity associated with aminoglycosides

3. Penicillin-allergic patients:

   • Vancomycin 30 mg/kg/day IV in two divided doses 2, 1
   • Duration: 6 weeks 2
   • Maintain serum trough concentration of 10-20 μg/mL 1

Special Considerations

Antibiotic Susceptibility

• E. faecalis is generally highly susceptible to amoxicillin (MIC90 = 0.75 μg/mL) 3
• 100% of periodontal E. faecalis isolates in one study were susceptible to amoxicillin/clavulanate 4
• Amoxicillin has demonstrated superior bactericidal activity compared to penicillin against E. faecalis 5

Resistance Patterns

• Test enterococci routinely for high-level resistance to gentamicin (MIC >500 μg/mL) 1
• If high-level aminoglycoside resistance is present, use double β-lactam therapy (ampicillin plus ceftriaxone) 1
• E. faecalis shows substantial resistance to tetracycline (53.2%), erythromycin (80.8%), clindamycin (100%), and metronidazole (100%) 4

Treatment Duration

• Endocarditis: 4-6 weeks 2, 1
• Bacteremia without endocarditis: 10-14 days after resolution of signs of infection 1
• Complicated UTI: 10-14 days 1
• Uncomplicated infections: 5-7 days 1

Prophylaxis for Endocarditis

• Single-dose oral amoxicillin (2g in US, 3g in UK) provides effective prophylaxis against E. faecalis endocarditis in moderate-risk patients 6
• For high-risk patients: IV amoxicillin (2g) or vancomycin (1g) plus gentamicin 6

Monitoring

• Assess clinical response within 48-72 hours of starting treatment 1
• For endocarditis, repeat blood cultures to confirm clearance of bacteremia 1
• Monitor renal function regularly, especially with aminoglycoside use 1

Remember that while E. faecalis is generally susceptible to amoxicillin, serious infections like endocarditis require combination therapy to achieve bactericidal activity and prevent treatment failure.