In an adult patient without a beta‑lactam allergy, is doxycycline an effective treatment for a urinary‑tract infection caused by Enterococcus faecalis?

Doxycycline for Enterococcus faecalis UTI

Doxycycline is not a recommended first-line agent for Enterococcus faecalis UTI in patients without beta-lactam allergy, and ampicillin or amoxicillin should be used instead. 1

First-Line Treatment for E. faecalis UTI

Ampicillin is the drug of choice for enterococcal infections, including E. faecalis UTI. 1 In patients without beta-lactam allergy:

• High-dose ampicillin (18-30 g IV daily in divided doses) or amoxicillin (500 mg PO/IV every 8 hours) should be used as first-line therapy. 1, 2
• High urinary concentrations of ampicillin can overcome even ampicillin-resistant E. faecalis in UTIs, achieving necessary bactericidal activity. 1
• One retrospective study showed clinical and microbiological eradication rates of 88.1% and 86%, respectively, in ampicillin-resistant VRE UTIs treated with ampicillin. 1

Doxycycline: Limited Role

While doxycycline has been mentioned in the literature for enterococcal infections, its role is extremely limited:

• Doxycycline has been used only as salvage therapy in combination with linezolid for vancomycin-resistant enterococcal infections, not as monotherapy. 1
• Doxycycline combined with bacitracin has been studied for suppressing VRE intestinal colonization, not for treating active UTIs. 3
• E. faecalis strains show high resistance rates to tetracyclines (96% in one study), making doxycycline unreliable. 4
• Doxycycline is mentioned as a possible oral option for VRE cystitis with intrinsic activity, but only when other options are unavailable. 5, 6

Recommended Treatment Algorithm for E. faecalis UTI

For patients without beta-lactam allergy:

1. First-line: Ampicillin or amoxicillin (regardless of reported susceptibility for UTI due to high urinary concentrations) 1, 2
2. Alternative oral options for uncomplicated cystitis: Nitrofurantoin (100 mg PO every 6 hours) or fosfomycin (3 g PO single dose) 1, 2, 5
3. For complicated UTI or pyelonephritis: IV ampicillin with or without gentamicin (if gentamicin-susceptible) 1

For patients with true penicillin allergy:

1. First-line: Vancomycin (30 mg/kg/day IV in two divided doses, adjusted to trough 10-20 μg/mL) 7
2. Alternative for uncomplicated cystitis: Nitrofurantoin or fosfomycin 7, 2

Critical Pitfalls to Avoid

• Do not use fluoroquinolones for E. faecalis UTI—resistance rates are extremely high (46-47% for ciprofloxacin). 7, 4
• Do not use cephalosporins as monotherapy—enterococci are intrinsically resistant. 7, 2
• Do not assume vancomycin susceptibility without testing—always obtain susceptibility testing. 7
• Do not treat asymptomatic bacteriuria with E. faecalis—differentiate colonization from true infection. 1, 5

When Doxycycline Might Be Considered

Doxycycline should only be considered in the rare scenario where:

• The patient has vancomycin-resistant E. faecalis (VRE)
• The patient cannot tolerate beta-lactams, nitrofurantoin, or fosfomycin
• Susceptibility testing confirms doxycycline activity
• Even then, it should be used in combination therapy, not as monotherapy 1, 3

Bottom line: In an adult patient without beta-lactam allergy, ampicillin or amoxicillin—not doxycycline—is the appropriate treatment for E. faecalis UTI. 1, 2