What is the recommended treatment for an adult urinary tract infection caused by Enterococcus, including first‑line oral therapy and alternatives for complicated infection or resistant strains?

Treatment of Enterococcus Urinary Tract Infections

Ampicillin or amoxicillin is the drug of choice for enterococcal UTIs, with oral amoxicillin 500 mg every 8 hours for 7 days as first-line therapy for uncomplicated infections, achieving 88% clinical cure rates. 1

First-Line Oral Therapy for Uncomplicated UTI

For susceptible Enterococcus faecalis (the most common species):

• Amoxicillin 500 mg orally every 8 hours for 7 days is the preferred first-line agent, with clinical eradication rates of 88.1% and microbiological eradication of 86% 1
• Ampicillin 500 mg orally every 8 hours for 7 days is an equivalent alternative 1
• These agents remain effective even when in vitro testing suggests resistance, because high urinary concentrations can overcome elevated MICs 1, 2

Alternative oral agents when ampicillin/amoxicillin cannot be used:

• Nitrofurantoin 100 mg orally every 6 hours for 7 days has good activity against E. faecalis with resistance rates below 6% 1, 3
• Fosfomycin 3 g orally as a single dose is FDA-approved specifically for E. faecalis UTI and recommended for uncomplicated infections 1, 4, 5

Critical Treatment 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:

• Asymptomatic bacteriuria with Enterococcus does not routinely require treatment 1, 4
• The primary reason for exclusion from treatment in one study was asymptomatic bacteriuria (64% of cases) 6
• Look for specific UTI symptoms: dysuria, frequency, urgency, suprapubic pain, or systemic symptoms (fever >37.8°C, rigors, costovertebral angle tenderness) 1

Treatment Duration Based on Infection Severity

Uncomplicated cystitis: 7 days of therapy 1

Extend to 10-14 days if complicating factors present:

• Upper tract involvement (costovertebral angle tenderness) 1
• Systemic symptoms (fever, rigors, delirium) 1
• Urinary retention or obstruction 1

Agents to Avoid

Fluoroquinolones should be avoided due to high resistance rates (46-47% for ciprofloxacin/levofloxacin) 1

Cephalosporins are ineffective as enterococci have natural resistance to all cephalosporins 1

Nitrofurantoin limitations:

• Should NOT be used for complicated UTI, pyelonephritis, or systemic enterococcal infections (bacteremia, endocarditis, intra-abdominal infections) due to poor tissue and serum concentrations 1, 4
• Contraindicated in patients with creatinine clearance <60 mL/min 1

Treatment for Complicated or Hospitalized Patients

For patients requiring IV therapy:

• High-dose ampicillin 18-30 g IV daily in divided doses or amoxicillin 500 mg IV every 8 hours 1
• Piperacillin-tazobactam is an acceptable alternative based on susceptibility 7, 1
• Vancomycin can be used for susceptible strains in penicillin-allergic patients 7

For health care-associated infections, empiric anti-enterococcal therapy should target E. faecalis and include:

• Ampicillin, piperacillin-tazobactam, or vancomycin based on susceptibility 7
• This is particularly important for postoperative patients, those previously receiving cephalosporins, immunocompromised patients, and those with valvular heart disease or prosthetic materials 7

Vancomycin-Resistant Enterococcus (VRE) UTI

For ampicillin-susceptible VRE strains:

• Ampicillin remains effective despite vancomycin resistance, as high urinary concentrations overcome elevated MICs 1, 4, 2

For ampicillin-resistant VRE:

Lower UTI (cystitis):

• Nitrofurantoin 100 mg orally every 6 hours for 7 days 4, 5
• Fosfomycin 3 g orally single dose 4, 5
• Doxycycline (if susceptible) 4

Upper UTI or bacteremic VRE:

• Linezolid should be reserved for confirmed or suspected upper tract/bacteremic VRE UTIs among ampicillin-resistant strains 1, 4
• Linezolid appears as effective as comparator antibiotics for mild VRE UTI (9% vs 5% re-initiation of antibiotics, p=0.56) 6
• Daptomycin 8-12 mg/kg/day IV may be used for severe VRE UTIs with bacteremia 1, 4, 5

Empiric therapy directed against vancomycin-resistant E. faecium is NOT recommended unless the patient is at very high risk (e.g., liver transplant recipient with hepatobiliary infection or known VRE colonization) 7

Common Pitfalls to Avoid

• Do not treat asymptomatic bacteriuria – this is the most common error and drives unnecessary antibiotic use 1, 4, 6
• Do not use vancomycin orally for UTI – it has no oral formulation that achieves adequate urinary concentrations 1
• Do not use nitrofurantoin for pyelonephritis or bacteremia – tissue penetration is inadequate 1, 4
• Do not assume in vitro ampicillin resistance means clinical failure – urinary concentrations may still be therapeutic 1, 2
• Do not forget to remove indwelling urinary catheters when present, as this improves outcomes 5