What is the recommended treatment for a patient with a urinary tract infection (UTI) caused by Enterococcus faecalis using oral ampicillin?

Oral Ampicillin for UTI with Enterococcus faecalis

Ampicillin 500 mg orally every 8 hours for 7 days is the first-line treatment for uncomplicated urinary tract infections caused by Enterococcus faecalis, achieving clinical cure rates of 88% and microbiological eradication rates of 86%. 1

First-Line Oral Treatment

• Ampicillin remains the drug of choice for enterococcal UTIs, with the standard oral regimen being 500 mg every 8 hours for 7 days 1
• Amoxicillin 500 mg orally every 8 hours for 7 days is an equivalent alternative with similar efficacy 1
• The FDA specifically approves oral ampicillin for genitourinary tract infections caused by enterococci 2

Critical Advantage of Ampicillin

• High urinary concentrations of ampicillin can overcome elevated MICs even in ampicillin-resistant strains, making it effective when in vitro susceptibility testing suggests resistance 1
• This unique pharmacokinetic property makes ampicillin superior to other agents for enterococcal UTIs, as the drug achieves sufficient urinary concentrations that exceed typical resistance thresholds 1

Alternative Oral Agents (When Ampicillin Cannot Be Used)

• Nitrofurantoin 100 mg orally every 6 hours for 7 days is an effective alternative with resistance rates below 6% against E. faecalis 1

  • Critical limitation: Do not use nitrofurantoin if creatinine clearance is <60 mL/min, as urinary concentrations become inadequate 1
  • Do not use for complicated UTI or pyelonephritis, as it achieves poor tissue and serum concentrations 1

• Fosfomycin 3 g orally as a single dose is FDA-approved specifically for UTI caused by E. faecalis and is recommended for uncomplicated infections 1

Agents to Avoid

• Fluoroquinolones should be avoided due to high resistance rates (46-47% for ciprofloxacin/levofloxacin) in E. faecalis 1, 3
• All cephalosporins must be avoided, as E. faecalis has natural resistance to all cephalosporins 1

Essential Clinical Considerations

• Always obtain susceptibility testing before initiating therapy, even for strains described as "pansensitive," as resistance patterns vary significantly by institution 1
• Differentiate colonization from true infection before prescribing antibiotics, as asymptomatic bacteriuria with E. faecalis does not routinely require treatment 1
• For beta-lactamase producing strains, replace ampicillin with amoxicillin-clavulanate 500 mg every 8 hours for 7 days 1

Treatment Duration

• 7 days is the standard duration for uncomplicated UTI 1
• Longer durations may be necessary for complicated UTI or pyelonephritis, though treatment should be based on clinical response 1

When to Escalate to Parenteral Therapy

• If the patient requires hospitalization or has complicated UTI/pyelonephritis, use high-dose ampicillin 18-30 g IV daily in divided doses or amoxicillin 500 mg IV every 8 hours 1
• For vancomycin-resistant E. faecalis with severe UTI and bacteremia, consider daptomycin 8-12 mg/kg/day IV or linezolid for confirmed upper tract infections 1