Most Likely Causative Organism of Urinary Tract Infections
Escherichia coli is the most likely culprit, causing approximately 75% of urinary tract infections across all patient groups. 1
Primary Pathogen
- E. coli accounts for the vast majority of UTIs, representing approximately 75% of recurrent UTIs and up to 90% of all UTIs in ambulatory populations 1, 2
- E. coli is the most common organism in both uncomplicated and complicated UTIs across all patient demographics 1
- Most uropathogenic E. coli belong to phylogenetic lineage B2 and possess specific virulence factors including P-fimbriae, S or Dr adhesins, hemolysin, and cytotoxic necrotizing factor 1 2
Other Common Pathogens
After E. coli, the following organisms account for most remaining UTIs:
- Enterococcus faecalis - second most common pathogen 1, 3
- Proteus mirabilis - particularly in patients with risk factors for complicated UTIs 1, 3
- Klebsiella species - more common in complicated infections and healthcare settings 1, 4, 5
- Staphylococcus saprophyticus - especially in young, sexually active women 1, 3
Context-Specific Considerations
Complicated UTIs
In patients with structural abnormalities, instrumentation, diabetes, or immunosuppression, the microbial spectrum broadens beyond E. coli to include multiresistant nosocomial Gram-negative bacilli, Enterococcus species, and yeasts 1, 6
Healthcare-Associated UTIs
Catheter-associated UTIs are predominantly caused by multiresistant nosocomial Gram-negative bacilli (other than E. coli), Enterococcus species, and Candida 1
Pseudomonas aeruginosa
This organism should be suspected specifically in complicated UTIs with urinary tract obstruction, indwelling catheters, recent instrumentation, or healthcare-associated infections 7
Clinical Implications
- Empiric therapy should target E. coli as the primary pathogen in community-acquired uncomplicated UTIs 1, 4
- Trimethoprim-sulfamethoxazole is FDA-approved for UTIs caused by susceptible E. coli, Klebsiella, Enterobacter, Morganella morganii, Proteus mirabilis, and Proteus vulgaris 4
- Amoxicillin-clavulanate targets beta-lactamase-producing isolates of E. coli, Klebsiella, and Enterobacter species 5
- Urine culture should be obtained when symptoms don't resolve, recur within 4 weeks, in atypical presentations, suspected pyelonephritis, or pregnancy 1