Oral Ampicillin for VRE Urinary Tract Infections
Yes, ampicillin is available as an oral antibiotic and remains the drug of choice for VRE urinary tract infections in elderly patients, even when in vitro testing suggests resistance, because high urinary concentrations overcome elevated MICs. 1, 2
Oral Ampicillin Formulations and Dosing
Ampicillin is FDA-approved in oral formulations (capsules and oral suspension) for genitourinary tract infections caused by enterococci. 2
For uncomplicated VRE UTIs, ampicillin 500 mg orally every 8 hours for 7 days achieves clinical eradication rates of 88.1% and microbiological eradication of 86%. 1
Amoxicillin 500 mg orally every 8 hours for 7 days is an equivalent alternative with similar efficacy. 1
Why Ampicillin Works Despite Resistance
High urinary concentrations of ampicillin can overcome the high ampicillin MIC of ampicillin-resistant VRE, achieving necessary drug concentrations for optimal bactericidal activity in urinary tract infections. 3
This makes ampicillin effective even when laboratory susceptibility testing suggests resistance—a critical clinical pearl that distinguishes UTI treatment from other infection sites. 1
One retrospective study of ampicillin-resistant VRE UTIs treated with ampicillin showed 88.1% clinical success and 86% microbiological eradication. 3
Alternative Oral Options
If ampicillin cannot be used, consider these alternatives:
Fosfomycin 3 g orally as a single dose is FDA-approved specifically for E. faecalis UTI and is recommended for uncomplicated infections. 3, 1
Nitrofurantoin 100 mg orally every 6 hours for 7 days has good in vitro activity against VRE with resistance rates below 6%. 1
Critical Caveats for Elderly Patients
Always differentiate colonization from true infection before prescribing antibiotics—asymptomatic bacteriuria with VRE does not require treatment in elderly patients. 1, 4
Obtain susceptibility testing before initiating therapy, even for strains described as "pansensitive," as resistance patterns vary significantly. 1
Nitrofurantoin should not be used if creatinine clearance is <60 mL/min, as urinary concentrations become inadequate—particularly important in elderly patients with reduced renal function. 1
Nitrofurantoin is contraindicated for complicated UTIs or pyelonephritis due to poor tissue and serum concentrations. 1
When Oral Therapy Is Insufficient
For hospitalized patients requiring IV therapy, high-dose ampicillin (18-30 g IV daily in divided doses) or amoxicillin 500 mg IV every 8 hours is recommended. 1
For serious VRE infections (bacteremia, endocarditis), daptomycin at 8-10 mg/kg or linezolid should be considered, but these are not first-line for uncomplicated UTI. 3