What is the recommended treatment for a urinary tract infection caused by Proteus mirabilis with a colony-forming unit (CFU) count greater than 100,000 CFU/mL?

Treatment for Urinary Tract Infection Caused by Proteus mirabilis

For this urinary tract infection with Proteus mirabilis (>100,000 CFU/mL), treatment with ciprofloxacin or levofloxacin is recommended based on the susceptibility testing results showing excellent sensitivity to these agents.

Analysis of Susceptibility Results

The urine culture shows a significant growth of Proteus mirabilis with a colony count exceeding 100,000 CFU/mL, which is diagnostic of a urinary tract infection. The susceptibility testing reveals:

Susceptible Antibiotics:

• Ampicillin/Sulbactam (S, MIC ≤2)
• Cefepime (S, MIC 0.5)
• Ceftriaxone (S, MIC ≤0.25)
• Ciprofloxacin (S, MIC ≤0.06)
• Gentamicin (S, MIC ≤1)
• Levofloxacin (S, MIC ≤0.12)
• Meropenem (S, MIC 1)
• Piperacillin/Tazobactam (S, MIC ≤4)
• Trimethoprim/Sulfamethoxazole (S, MIC ≤20)

Intermediate/Resistant Antibiotics:

• Cefazolin (I, MIC 4)
• Ceftazidime (R, MIC ≥32)
• Nitrofurantoin (R, MIC 128)

Treatment Recommendation Algorithm

1. First-line treatment options (based on susceptibility and current guidelines 1):

   • Ciprofloxacin 500 mg PO BID for 7 days
   • Levofloxacin 750 mg PO daily for 5 days

2. Alternative options (if fluoroquinolones are contraindicated):

   • Trimethoprim/Sulfamethoxazole 160/800 mg PO BID for 7-14 days
   • Ceftriaxone 1-2 g IV/IM daily (if parenteral therapy needed)

3. Considerations for complicated UTI (if applicable):

   • Extend treatment duration to 7-14 days
   • Consider initial IV therapy if systemic symptoms present

Rationale for Recommendation

Fluoroquinolones (ciprofloxacin or levofloxacin) are recommended as first-line therapy because:

1. The organism shows excellent susceptibility (very low MICs)
2. These agents achieve high urinary concentrations
3. They have good activity against Proteus mirabilis
4. They can be administered orally with excellent bioavailability
5. The European Association of Urology guidelines support their use for UTIs caused by susceptible organisms 1

Important Clinical Considerations

• Proteus mirabilis is a urease-producing organism that can cause urinary stones and encrustation of catheters 2, 3
• Assess for any underlying urological abnormalities that may complicate treatment 1
• If the patient has indwelling catheters, consider catheter change during treatment
• Nitrofurantoin should be avoided despite its common use in uncomplicated UTIs, as it shows resistance in this case and is generally ineffective against Proteus species 1
• Cefazolin shows intermediate susceptibility and should not be used as monotherapy

Monitoring and Follow-up

• Clinical improvement should occur within 48-72 hours of appropriate therapy
• Consider follow-up urine culture in complicated cases or if symptoms persist
• Evaluate for any anatomical abnormalities or foreign bodies (stones) if recurrent infections with Proteus mirabilis occur

Pitfalls to Avoid

1. Using nitrofurantoin despite susceptibility results showing resistance
2. Treating for too short a duration if this represents a complicated UTI
3. Failing to address any underlying anatomical or functional abnormalities
4. Not changing catheters (if present) during treatment
5. Overlooking the potential for stone formation with Proteus infections

The high colony count and the organism's ability to form biofilms and stones necessitate thorough treatment to prevent complications such as pyelonephritis and septicemia 4.