Can amoxicillin be used to treat Enterococcus faecalis urinary tract infections?

Amoxicillin for Enterococcus faecalis UTI

Amoxicillin 500 mg orally every 8 hours for 7 days is the first-line treatment for uncomplicated urinary tract infections caused by Enterococcus faecalis, with clinical eradication rates of 88.1% and microbiological eradication rates of 86%. 1, 2

FDA-Approved Indication

• Amoxicillin is FDA-approved specifically for genitourinary tract infections caused by susceptible (β-lactamase-negative) isolates of Enterococcus faecalis 3
• The drug should be taken at the start of a meal to minimize gastrointestinal intolerance 3

Dosing and Duration

• Standard oral dosing: Amoxicillin 500 mg every 8 hours for 7 days 1, 2
• Intravenous dosing (for hospitalized patients): Ampicillin 2 g IV every 4 hours 1
• High-dose regimen (for VRE with high MIC): Amoxicillin 500 mg IV every 8 hours can achieve sufficient urinary concentrations to overcome ampicillin-resistant VRE 4

Why Amoxicillin Over Other Options

• Amoxicillin achieves MICs two to four times lower than ampicillin against enterococci, making it the preferred aminopenicillin 4
• Aminopenicillins remain the drug of choice for enterococcal infections when organisms are susceptible 5
• High urinary concentrations of amoxicillin can overcome elevated MICs in ampicillin-resistant VRE strains 4

Alternative First-Line Options

• Ampicillin: 500 mg orally every 8 hours for 7 days (equivalent efficacy to amoxicillin) 1, 2
• Fosfomycin: 3 g single oral dose (FDA-approved specifically for E. faecalis UTIs, offers convenient single-dose therapy) 1, 2
• Nitrofurantoin: 100 mg orally every 6 hours for 7 days (effective alternative with good in vitro activity and low resistance rates) 1, 2

Treatment for Vancomycin-Resistant E. faecalis (VRE)

• For uncomplicated VRE UTI: Fosfomycin 3 g single oral dose OR nitrofurantoin 100 mg every 6 hours 1
• For ampicillin-resistant VRE: High-dose amoxicillin (500 mg IV every 8 hours) can still be effective due to high urinary drug concentrations 4
• Clinical and microbiological eradication rates of 88.1% and 86% respectively have been documented even in ampicillin-resistant VRE UTIs treated with ampicillin 4

Critical Caveats

• Avoid fluoroquinolones: Ciprofloxacin and levofloxacin show 46-47% resistance rates in E. faecalis UTIs and carry unfavorable risk-benefit profiles per FDA warnings 1, 2, 6
• Confirm susceptibility testing: Even for "pansensitive" strains, susceptibility testing should be performed before initiating therapy 1, 2
• β-lactamase production: Amoxicillin is only effective against β-lactamase-negative isolates; if β-lactamase production is present, use amoxicillin-clavulanate instead 4, 3

Resistance Patterns

• E. faecalis demonstrates 96-100% susceptibility to penicillin/amoxicillin in most studies 7
• E. faecium shows significantly lower susceptibility (only 32%) to penicillin, making it a more challenging pathogen 7
• High-level aminoglycoside resistance (HLAR) occurs in 17% of E. faecalis strains, eliminating synergistic combination options 7

Treatment Algorithm

1. Obtain urine culture and susceptibility testing 1, 2
2. For susceptible E. faecalis (uncomplicated UTI): Start amoxicillin 500 mg every 8 hours for 7 days 1, 2
3. For penicillin allergy or intolerance: Use nitrofurantoin 100 mg every 6 hours for 7 days 1, 2
4. For VRE (uncomplicated UTI): Use fosfomycin 3 g single dose OR nitrofurantoin 1
5. For complicated UTI or pyelonephritis: Consider IV ampicillin 2 g every 4 hours or longer treatment duration 1

Special Considerations for Severe Infections

• For bacteremia or severe infections with VRE, daptomycin 8-12 mg/kg/day should be considered instead of oral agents 4, 1
• Combination therapy with ampicillin plus gentamicin may be warranted for complex infections with high inoculum, provided the strain does not have HLAR 4
• Treatment duration should extend at least 48-72 hours beyond symptom resolution 3