What is the preferred antibiotic for a patient with a urinary tract infection caused by Enterococcus faecalis, as indicated by a urine culture, considering potential penicillin allergy and impaired renal function?

Preferred Antibiotic for Enterococcus faecalis Urinary Tract Infection

Amoxicillin 500 mg orally every 8 hours for 7 days is the first-line treatment for uncomplicated E. faecalis UTI, achieving 88.1% clinical and 86% microbiological eradication rates. 1, 2

First-Line Treatment: Ampicillin/Amoxicillin

Ampicillin or amoxicillin remains the drug of choice for enterococcal UTIs, even when in vitro testing suggests resistance, because high urinary concentrations can overcome elevated MICs. 1, 2

Dosing Regimens:

• Oral therapy: Amoxicillin 500 mg every 8 hours for 7 days (or ampicillin 500 mg every 8 hours for 7 days as equivalent alternative) 1, 2
• IV therapy for hospitalized patients: Ampicillin 2 g IV every 4 hours, or high-dose ampicillin 18-30 g IV daily in divided doses to achieve sufficient urinary concentrations 1, 2

Key Advantage:

• High urinary concentrations of ampicillin can overcome high ampicillin MICs even in ampicillin-resistant VRE strains, making it effective when susceptibility testing suggests resistance 1

Alternative Oral Agents for Uncomplicated UTI

Nitrofurantoin

• Dosing: 100 mg orally every 6 hours for 7 days 1, 2
• Activity: Good in vitro activity against E. faecalis with resistance rates below 6% 1, 2
• Appropriate for: Penicillin-allergic patients with uncomplicated cystitis 1

Fosfomycin

• Dosing: 3 g orally as single dose 1, 2
• FDA-approved specifically for E. faecalis UTI 1, 2
• Convenient single-dose therapy for uncomplicated infections 1, 2

Critical Contraindications and Limitations

Nitrofurantoin Should NOT Be Used For:

• Complicated UTIs or pyelonephritis (achieves poor tissue and serum concentrations) 1
• Creatinine clearance <60 mL/min (urinary concentrations become inadequate) 1
• Systemic enterococcal infections (bacteremia, endocarditis, intra-abdominal infections) 1

Fluoroquinolones Should Be Avoided:

• High resistance rates: 46-47% for ciprofloxacin/levofloxacin 2, 3
• Unfavorable risk-benefit profile for uncomplicated UTIs 2
• Ciprofloxacin is no longer recommended for E. faecalis complicated UTI in men with risk factors 3

Special Considerations for Penicillin Allergy

For patients with penicillin allergy and normal renal function, nitrofurantoin 100 mg orally every 6 hours for 7 days is the appropriate alternative for uncomplicated cystitis. 1, 2

• For complicated UTI or pyelonephritis in penicillin-allergic patients, parenteral options become necessary (see below) 1

Management with Impaired Renal Function

If CrCl ≥60 mL/min:

• Amoxicillin/ampicillin remains first-line 1, 2
• Nitrofurantoin is acceptable 1, 2

If CrCl <60 mL/min:

• Avoid nitrofurantoin (inadequate urinary concentrations) 1
• Use amoxicillin/ampicillin with dose adjustment 1, 2
• Fosfomycin 3 g single dose remains an option 1, 2

Treatment for Vancomycin-Resistant E. faecalis (VRE)

Uncomplicated VRE UTI:

• First choice: Fosfomycin 3 g PO single dose 2
• Alternative: Nitrofurantoin 100 mg PO every 6 hours (if CrCl ≥60 mL/min) 2
• High-dose ampicillin may still be effective due to high urinary concentrations overcoming resistance 1

Complicated VRE UTI or Pyelonephritis:

• Daptomycin 8-12 mg/kg/day IV for severe infections with bacteremia 2
• Linezolid should be reserved for confirmed or suspected upper tract/bacteremic VRE UTIs among ampicillin-resistant strains 4

Essential Clinical Approach

Always Obtain Susceptibility Testing:

• Confirm susceptibility testing before initiating therapy, even for strains described as "pansensitive" 1, 2
• Resistance patterns vary significantly by institution and patient population 1

Differentiate Colonization from Infection:

• Asymptomatic bacteriuria with E. faecalis does not routinely require treatment 1, 4
• Unnecessary antibiotic use in colonized patients has become a major problem in hospitals and long-term care facilities 4

Treatment Duration

• Uncomplicated UTI: 7 days 1, 2
• Complicated UTI or pyelonephritis: Longer durations may be necessary based on clinical response 1, 2

Common Pitfalls to Avoid

• Do not use cephalosporins: Enterococci have natural resistance to all cephalosporins 5
• Do not rely on fluoroquinolones empirically: Nearly half of E. faecalis strains are resistant 2, 3
• Do not use nitrofurantoin for pyelonephritis or in renal impairment: Poor tissue penetration and inadequate urinary levels with CrCl <60 mL/min 1
• Do not treat asymptomatic bacteriuria: Reserve antibiotics for symptomatic infections 1, 4