Emery Haley, PhD, Scientific Writing Specialist
Citrobacter koseri
Clinical Summary
- C. koseri is recognized as a classical gram-negative, urease-variable, biofilm-forming uropathogen.
- C. koseri is associated with complicated UTI in older adult and pediatric populations.
- In symptomatic UTI patients, C. koseri:
- Is not a contaminant (is found in catheter-collected urine specimens).
- Is viable (can grow out on culture).
- Is pathogenic (associated with elevated urine biomarkers of infection).
- Reported severe complications of C. koseri UTI include pyelonephritis, polymicrobial bacteremia, and urosepsis.
- Multidrug-resistant C. koseri (especially among hospital-acquired UTIs) is a significant global health threat.
Bacterial Characteristics
Gram-stain
Gram-negative
Morphology
Bacillus
Growth Requirements
Non-fastidious (grows well in standard urine culture conditions)
Facultative anaerobe
Nitrate Reduction
Yes
Urease
Variable
Biofilm Formation
Yes
Pathogenicity
Colonizer or Pathobiont
Clinical Relevance in UTI
C. koseri is a urease-variable microorganism [1] capable of forming biofilms. C. koseri is typically considered to be among the common classical uropathogens in older adults,[2] but is also being increasingly recognized as common in pediatric UTIs.[3]
In a study of older adult males and females with clinically suspected complicated UTI, C. koseri was detected in both midstream voided and in-and-out-catheter collected specimens indicating that it was truly present in the bladder, not simply a contaminant picked up during voiding.[4] Furthermore, elevated markers of immune system activation in the urinary tract have been measured from the same clinical urine specimens in which C. koseri was detected, indicating that the presence of C. koseri was associated with an immune response to urinary tract infection.[5–7] Hospital-acquired C. koseri UTIs pose a particularly serious threat, as these infections increasingly display multidrug- or pandrug-resistance.[8] Severe complications of C. koseri UTI, including pyelonephritis, polymicrobial bacteremia, and urosepsis, have been reported.[9–11]
Hospital-acquired C. koseri UTIs pose a particularly serious threat, as these infections increasingly display multidrug- or pandrug-resistance.[8] Although not considered one of the six highest priority “ESKAPE” pathogens, The World Health Organization (WHO) has included this organism in the 2024 Bacterial Priority Pathogens List (BPPL).[111] Severe complications of C. koseri UTI, including pyelonephritis, polymicrobial bacteremia, and urosepsis, have been reported.[9–11]
Together, these findings indicate that C. koseri should be seriously considered as a uropathogen and demonstrate the value of detecting this organism, particularly in individuals with recurrent UTI and/or risk factors for persistent or complicated UTI.
Treatment
Evidence of Efficacy (Checkmarks): Amoxicillin/Clavulanate, Ampicillin/Sulbactam, Cefaclor, Cefazolin, Cefepime, Ceftazidime, Ceftriaxone, Ciprofloxacin, Gentamicin, Levofloxacin, Meropenem, Nitrofurantoin, Piperacillin/Tazobactam, Sulfamethoxazole/Trimethoprim, and Trimethoprim.