Oral Amoxicillin Dosing for UTI Caused by Enterococcus faecalis
For a urinary tract infection caused by Enterococcus faecalis, amoxicillin 500 mg orally every 8 hours is the recommended dosing regimen. 1
Treatment Rationale
Enterococcus faecalis remains generally susceptible to ampicillin/amoxicillin, making it the preferred first-line treatment for uncomplicated UTIs caused by this organism. The 2022 guidelines for treatment of infections due to multidrug resistant organisms specifically recommend amoxicillin 500 mg IV or PO every 8 hours for uncomplicated urinary tract infections due to enterococci 1.
Mechanism of Action
Amoxicillin works by:
- Inhibiting bacterial cell wall synthesis
- Achieving high urinary concentrations that can overcome even relatively high MICs
- Maintaining bactericidal activity against most E. faecalis strains
Pharmacokinetic Considerations
- For beta-lactams like amoxicillin, efficacy depends on time above MIC (T>MIC)
- Studies show that a cumulative T>MIC of approximately 30 hours is necessary for maximal cure rates of 80-90% 2
- Amoxicillin achieves high urinary concentrations that can often overcome even isolates with elevated MICs 3
Alternative Options
If the E. faecalis isolate is resistant to amoxicillin or the patient has a penicillin allergy, consider:
Nitrofurantoin 100 mg PO every 6 hours 1
- Excellent activity against E. faecalis
- 100% susceptibility reported in some studies 4
- Limited to lower UTI treatment only
Fosfomycin 3 g PO as a single dose 1
- Convenient single-dose therapy
- FDA-approved for UTI caused by E. faecalis
- Good option for uncomplicated lower UTI
Special Considerations
Antimicrobial Resistance
- E. faecalis generally remains highly susceptible to amoxicillin (96% susceptibility in some studies) 4
- This contrasts with E. faecium, which has much lower susceptibility to penicillins (32% susceptibility) 4
- Always check susceptibility results before finalizing treatment
High-Level Aminoglycoside Resistance (HLAR)
- Approximately 17% of E. faecalis strains may demonstrate HLAR 4
- This is important if combination therapy is being considered for complicated infections
- HLAR strains are often resistant to multiple other antibiotics
Duration of Therapy
- For uncomplicated lower UTI: 5-7 days is typically sufficient
- For complicated UTI or pyelonephritis: 10-14 days may be required
Common Pitfalls to Avoid
Confusing E. faecalis with E. faecium
- E. faecium has much higher rates of resistance to amoxicillin
- Always confirm species identification and susceptibility
Undertreating complicated UTIs
- Patients with obstructive uropathy, urological abnormalities, or immunosuppression may need longer therapy or different agents
- Male sex, obstructive uropathy, and nosocomial infection are risk factors for enterococcal UTI 5
Treating asymptomatic bacteriuria
- Routine treatment of asymptomatic bacteriuria with enterococci is not recommended
- Consider removing indwelling catheters if present 6
Inadequate dosing
- The recommended dose of amoxicillin (500 mg every 8 hours) is necessary to maintain adequate time above MIC
- Lower doses or extended intervals may lead to treatment failure
In summary, oral amoxicillin 500 mg every 8 hours is the treatment of choice for E. faecalis UTI, with nitrofurantoin and fosfomycin as reasonable alternatives for uncomplicated lower UTI when amoxicillin cannot be used.