What is the recommended treatment for Citrobacter koseri (C. koseri) in a urine culture?

Treatment of Citrobacter koseri in Urine Culture

Obtain a urine culture with susceptibility testing before initiating treatment, then use first-line agents (nitrofurantoin, trimethoprim-sulfamethoxazole, or fosfomycin) if the organism is susceptible, or fluoroquinolones/cephalosporins based on local resistance patterns and culture results for 7 days in uncomplicated cases. 1

Initial Diagnostic Approach

Always obtain urine culture and susceptibility testing prior to initiating antimicrobial therapy because Citrobacter koseri has inherent resistance to ampicillin and emerging resistance to beta-lactams and aminoglycosides. 1, 2, 3 This is particularly critical as C. koseri demonstrates variable resistance patterns, with historical data showing 100% resistance to ampicillin and carbenicillin, though only 9.3% resistance to other agents tested. 4

Determine if this is:

• Uncomplicated UTI: No structural/functional urinary tract abnormalities, no catheter, no immunosuppression
• Complicated UTI: Presence of obstruction, foreign body, catheter, male sex, pregnancy, diabetes, immunosuppression, or healthcare-associated infection 1

Treatment Selection Based on Clinical Context

For Uncomplicated Cystitis (Lower UTI)

Use first-line agents based on susceptibility results: 1

• Nitrofurantoin (preferred when susceptible due to low resistance rates) 1
• Trimethoprim-sulfamethoxazole (if organism susceptible) 1
• Fosfomycin (single 3g dose, though may have inferior efficacy) 1

Treatment duration: 7 days maximum for uncomplicated cases. 1

For Complicated UTI or Pyelonephritis

Empiric therapy should include: 1

• Fluoroquinolones (ciprofloxacin 500mg twice daily for 7 days OR levofloxacin 750mg daily for 5 days) if local resistance <10% 1
• Third-generation cephalosporins (ceftriaxone 1g IV) plus aminoglycoside for empiric coverage if fluoroquinolone resistance >10% or patient is severely ill 1
• Carbapenems may be required given emerging beta-lactam resistance in C. koseri 2, 3

Treatment duration: 7-14 days depending on clinical response (7 days for prompt resolution, 10-14 days for delayed response). 1

For Catheter-Associated UTI

Critical management steps: 1

1. Replace the catheter if it has been in place ≥2 weeks before initiating antimicrobial therapy to improve outcomes and reduce recurrence 1
2. Obtain urine culture from the freshly placed catheter prior to starting antibiotics 1
3. Remove catheter entirely if possible 1

Antimicrobial selection:

• Fluoroquinolones remain effective, with levofloxacin showing 79% microbiologic eradication in catheterized patients 1
• Expect polymicrobial infection and multidrug resistance 1

Treatment duration: 1

• 7 days for prompt symptom resolution
• 10-14 days for delayed response
• 5 days of levofloxacin acceptable for non-severely ill patients
• 3 days may be considered for women <65 years after catheter removal without upper tract symptoms

Critical Pitfalls and Considerations

Do NOT treat asymptomatic bacteriuria unless the patient is pregnant or undergoing urologic procedures with mucosal trauma. 1 Treatment of asymptomatic bacteriuria fosters antimicrobial resistance and increases recurrent UTI episodes. 1

Recognize C. koseri as a potentially serious pathogen: While typically causing uncomplicated UTIs (>50% of isolates from urinary tract), it can progress to life-threatening infections including peri-nephric abscess, empyema, sepsis, and even infective endocarditis, particularly in immunocompromised patients or those with structural abnormalities. 2, 5, 6

Avoid empiric ampicillin or amoxicillin as C. koseri has inherent resistance to these agents. 1, 4

Tailor therapy based on susceptibility results once available, as resistance patterns vary and emerging resistance to multiple drug classes is documented. 3, 4

Consider carbapenem therapy if the organism demonstrates resistance to cephalosporins and fluoroquinolones, as this is increasingly recognized. 2, 3