What are the causes of Urinary Tract Infections (UTIs)?

Causes of Urinary Tract Infections (UTIs)

Escherichia coli (E. coli) is the most common cause of urinary tract infections, responsible for approximately 75% of all UTIs, with other common pathogens including Enterococcus faecalis, Proteus mirabilis, Klebsiella species, and Staphylococcus saprophyticus. 1, 2

Primary Causative Organisms

Bacterial Pathogens

• E. coli: The predominant pathogen (75% of UTIs) 1
• Other common bacteria:
  • Enterococcus faecalis
  • Proteus mirabilis
  • Klebsiella species
  • Staphylococcus saprophyticus 1, 3, 4
• Less common bacteria:
  • Pseudomonas aeruginosa
  • Serratia marcescens
  • Citrobacter species
  • Enterobacter species 1

Non-bacterial Pathogens

• Fungi (rare)
• Viruses (rare) 3

Risk Factors and Contributing Causes

Anatomical Factors

• Urinary tract obstruction at any site 1
• Foreign bodies in the urinary tract 1
• Incomplete bladder emptying 1
• Vesicoureteral reflux 1
• Urethral or bladder diverticula 1
• Cystoceles (in postmenopausal women) 1

Host Factors

• Female gender: Shorter urethra provides easier access for bacteria 2
• Sexual activity: Especially with use of spermicides or diaphragms 1, 2
• Pregnancy 1
• Diabetes mellitus 1, 2
• Immunosuppression 1
• Postmenopausal status: Associated with urinary incontinence, cystocele, high postvoid residuals 1
• Genetic factors: Some individuals may have increased susceptibility 5

Iatrogenic Factors

• Catheterization: Leading cause of healthcare-associated UTIs 1
  • Risk increases 3-8% per day of catheterization 1
• Recent urinary tract instrumentation 1
• Prior urinary tract surgery 1

Bacterial Virulence Factors

• Adhesins (like fimbriae) that facilitate attachment to urinary epithelium 6, 5
• Iron-acquisition systems 5
• Toxin production 5
• Biofilm formation capacity (associated with recurrent infections) 5
• Ability to invade and replicate within bladder cells, forming intracellular bacterial communities 5

Pathophysiology of UTI Development

1. Colonization: Bacteria from fecal flora colonize the periurethral area 1
2. Ascension: Bacteria ascend the urethra to the bladder 1
3. Attachment: Pathogens attach to the bladder epithelium using adhesins 6, 5
4. Invasion: Some bacteria (particularly UPEC) can invade bladder cells 5
5. Biofilm formation: Creates persistent reservoirs resistant to antibiotics and host defenses 5
6. Further ascension: In some cases, bacteria ascend to kidneys causing pyelonephritis 1

Types of UTIs Based on Causation

Uncomplicated UTIs

• Occur in individuals with normal urinary tract anatomy and function
• Typically caused by E. coli (80-90% of community-acquired cases) 3
• Usually respond well to appropriate empiric therapy 1

Complicated UTIs

Associated with:

• Structural or functional abnormalities of the urinary tract
• Presence of foreign bodies
• Immunocompromised status
• Pregnancy
• Diabetes
• Healthcare-associated infections
• Multidrug-resistant organisms 1

Recurrent UTIs

• Can be due to:
  • Reinfection: New infection with a different strain (more common)
  • Relapse: Recurrence with the same strain due to persistent reservoirs 1, 5
  • Bacterial persistence: Often related to anatomical abnormalities, calculi, or foreign bodies 1

Prevention Strategies Based on Causation

• Adequate hydration: Promotes frequent urination and bacterial clearance 1, 2
• Urge-initiated voiding: Prevents bacterial stasis 1, 2
• Post-coital voiding: Reduces bacterial ascension after sexual activity 1, 2
• Avoiding spermicidal products: These alter vaginal flora 1, 2
• Vaginal estrogen: For postmenopausal women with atrophic vaginitis 1, 2
• Minimizing catheter use: And prompt removal when no longer needed 1

Understanding the causes of UTIs is essential for appropriate prevention, diagnosis, and treatment strategies. The high prevalence of E. coli as the causative agent, along with the various risk factors and pathogenic mechanisms, provides a foundation for targeted interventions to reduce the burden of these common infections.