What is the recommended dosage of ampicillin (Ampicillin) for treating Enterococcus Faecalis (E. Faecalis) infections?

Ampicillin Dosing for Enterococcus Faecalis Infections

For Enterococcus faecalis infections, the recommended dosage of ampicillin is 2g IV every 4-6 hours for uncomplicated infections, with treatment duration of 7-14 days, and extending to 4-6 weeks for endocarditis or other deep-seated infections. 1

Dosing Considerations Based on Infection Type

Uncomplicated Infections

• Ampicillin: 2g IV every 4-6 hours for 7-14 days 1
• For urinary tract infections without systemic involvement, shorter courses (5-7 days) may be sufficient

Invasive Infections (Bacteremia, Endocarditis)

• Ampicillin: 2g IV every 4 hours (12g/day) for 4-6 weeks 2
• For native valve endocarditis: 4-6 weeks of therapy 2
• For prosthetic valve endocarditis: 6 weeks of therapy 2

Combination Therapy Options

Traditional Approach: Ampicillin plus Aminoglycoside

• Historically, ampicillin plus gentamicin was the standard regimen for serious E. faecalis infections
• However, this combination has significant limitations:
  • High risk of nephrotoxicity, especially in elderly patients or those with renal impairment 1
  • Ineffective against high-level aminoglycoside-resistant (HLAR) strains 3

Modern Approach: Ampicillin plus Ceftriaxone

• Ampicillin: 2g IV every 4 hours plus Ceftriaxone: 2g IV every 12 hours 3
• This combination is now preferred because:
  • Similar efficacy to ampicillin-gentamicin (no difference in mortality or treatment failure) 3
  • Significantly lower risk of adverse events (1% vs 25%, p<0.001) 3
  • Effective against both aminoglycoside-susceptible and HLAR strains 4, 3
  • No risk of nephrotoxicity compared to aminoglycoside regimens 3

Special Considerations

Penicillin Allergy

• For patients with non-anaphylactic penicillin allergy, consider penicillin desensitization 2
• If beta-lactams cannot be given:
  • Vancomycin: 30 mg/kg/day IV in 2-3 divided doses 2
  • For serious infections in penicillin-allergic patients, vancomycin plus gentamicin may be considered (if renal function permits)

High-Level Aminoglycoside Resistance (HLAR)

• Aminoglycosides should not be used if HLAR is present (MIC >500 mg/L) 2
• Ampicillin plus ceftriaxone is the preferred regimen for HLAR strains 4, 3

Renal Impairment

• Ampicillin-ceftriaxone is preferred over aminoglycoside-containing regimens 1, 3
• Dose adjustment of ampicillin may be needed in severe renal impairment

Monitoring Recommendations

• Clinical response should be assessed within 48-72 hours of starting treatment
• For endocarditis, blood cultures should be repeated to confirm clearance of bacteremia
• Regular monitoring of renal function, especially if aminoglycosides are used

Common Pitfalls to Avoid

• Underestimating treatment duration for invasive infections
• Using aminoglycosides in patients with pre-existing renal impairment
• Failing to identify high-level aminoglycoside resistance
• Not considering the possibility of endocarditis in patients with E. faecalis bacteremia
• Using monotherapy for serious or invasive E. faecalis infections

The ampicillin plus ceftriaxone combination has emerged as the preferred regimen for serious E. faecalis infections due to its efficacy against both aminoglycoside-susceptible and resistant strains, along with its superior safety profile compared to aminoglycoside-containing regimens.