What is the best antibiotic choice for a urinary tract infection caused by Enterococcus (E.) faecalis with sensitivities to ampicillin (<=2), nitrofurantoin (<=16), and vancomycin (1)?

Antibiotic Selection for E. faecalis Urinary Tract Infection

Ampicillin is the drug of choice for treating E. faecalis urinary tract infections when the organism is susceptible, as in this case with MIC ≤2. 1

Rationale for Antibiotic Selection

First-line Option: Ampicillin

• Ampicillin is specifically recommended as the drug of choice for enterococcal infections, including UTIs
• High urinary concentrations of ampicillin can effectively treat these infections with clinical and microbiological eradication rates of 88.1% and 86% respectively 1
• The susceptibility result (MIC ≤2) indicates that the E. faecalis isolate is sensitive to ampicillin
• Ampicillin is preferred over broader-spectrum antibiotics to minimize antimicrobial resistance development

Alternative Options

1. Nitrofurantoin

   • Good option for uncomplicated lower UTIs caused by E. faecalis (MIC ≤16 indicates susceptibility) 1, 2
   • 88% of enterococci isolates are susceptible to nitrofurantoin 2
   • Limitations:
     • Should only be used if creatinine clearance >30 mL/min 1
     • Not appropriate for pyelonephritis or complicated UTIs due to poor tissue penetration
     • Typically used for lower UTIs only

2. Vancomycin

   • While the isolate is susceptible (MIC 1), vancomycin should be reserved for more resistant infections 3
   • FDA labeling indicates vancomycin is primarily indicated for serious infections when other options have failed 3
   • For enterococcal infections, vancomycin is typically used in combination with an aminoglycoside 3
   • Should be preserved as a last-line agent to prevent development of vancomycin-resistant enterococci (VRE)

Clinical Decision Algorithm

1. For uncomplicated lower UTI:

   • First choice: Oral ampicillin if susceptible (as in this case)
   • Alternative if renal function normal (CrCl >30 mL/min): Nitrofurantoin
   • Reserve vancomycin for patients with severe beta-lactam allergies or treatment failures

2. For complicated UTI or pyelonephritis:

   • First choice: Ampicillin (IV initially if severe, then oral step-down)
   • Reserve vancomycin for true penicillin allergies or treatment failures

Important Considerations

• Duration of therapy:

  • Uncomplicated UTI: 5-7 days
  • Complicated UTI: 7-10 days
  • Pyelonephritis: 10-14 days 1

• Common pitfalls to avoid:

  • Using vancomycin unnecessarily contributes to development of resistant organisms
  • Using nitrofurantoin in patients with renal impairment (CrCl <30 mL/min) or for upper UTIs
  • Treating asymptomatic bacteriuria in elderly patients, which does not improve outcomes and contributes to antibiotic resistance 1
  • Failing to remove indwelling catheters when possible, which is an important adjunctive measure

• Monitoring:

  • Clinical response should be evident within 48-72 hours
  • Consider repeat urine culture for complicated cases or treatment failures

By selecting ampicillin for this susceptible E. faecalis UTI, you're using the most targeted therapy with excellent efficacy while preserving broader-spectrum agents for when they're truly needed.