What causes recurrent urinary tract infections (UTIs)?

Causes of Recurrent Urinary Tract Infections

Recurrent urinary tract infections (UTIs) are primarily caused by bacterial reinfection from external sources or bacterial persistence within the urinary tract, with sexual activity and hygiene being the most common contributing factors in most cases. 1

Definition and Classification

• Recurrent UTIs are defined as at least three episodes of infection within a 12-month period following complete clinical resolution of previous UTIs 1
• Two main mechanisms of recurrence:
  • Reinfection: New infection from outside the urinary tract (more common)
  • Bacterial persistence: Same organism persisting within the urinary tract 1, 2
• Reinfections occur more than 2 weeks after symptom resolution or are caused by a different pathogen
• Relapses (persistent infections) occur within 2 weeks of treatment with the same organism 1

Bacterial Factors

• Escherichia coli is the most common causative organism, responsible for approximately 75% of recurrent UTIs 1
• Other common organisms include Enterococcus faecalis, Proteus mirabilis, Klebsiella, and Staphylococcus saprophyticus 1
• Recent research has revealed that uropathogenic E. coli (UPEC) can:
  • Bind and replicate within the urothelium
  • Form intracellular bacterial communities
  • Create quiescent intracellular reservoirs that can cause recurrent UTIs 2
• Bacterial survival in the bladder after antibiotic treatment is a key factor in recurrence 3

Host Factors and Risk Conditions

Anatomical and Functional Factors

• Urinary tract obstruction or structural abnormalities:
  • Cystoceles, urethral or bladder diverticula
  • Foreign bodies (including catheters)
  • Urinary tract calculi
  • Fistulae 1, 4
• Incomplete bladder emptying:
  • High post-void residual urine volume
  • Voiding dysfunction 1, 5
• Vesicoureteral reflux 5

Physiological Factors

• Postmenopausal status:
  • Urinary incontinence
  • Atrophic vaginitis
  • Cystocele 1, 4
• Pregnancy 5
• Immunosuppression 4
• Diabetes mellitus 4
• Defects in pathogen recognition and urothelial barrier function 3
• Nonsecretor blood type 5

Behavioral Factors

• Sexual activity:
  • Frequent intercourse
  • Sequential anal and vaginal intercourse 4, 5
• Use of diaphragms and/or spermicides 1, 4
• Inadequate hydration 4
• Prolonged urine retention 4
• History of UTIs during childhood or premenopause 5
• Family history of UTIs 5

Special Considerations

Postmenopausal Women

• Increased risk due to:
  • Urinary incontinence
  • Cystocele
  • High post-void residual urine
  • Atrophic vaginitis 1
• Vaginal estrogen therapy is strongly recommended for prevention as it helps restore vaginal microbiome and reduces vaginal atrophy 4

Complicated UTIs

• Patients with bacterial cystitis recurring rapidly (within 2 weeks) after treatment
• Those with bacterial persistence without symptom resolution
• May require imaging to detect treatable conditions 1

Prevention Strategies

Non-Antibiotic Approaches (First-Line)

• Self-care measures:
  • Adequate hydration (2-3L daily)
  • Urge-initiated voiding
  • Post-coital voiding
  • Avoiding spermicidal contraceptives 1, 4
• For postmenopausal women:
  • Topical vaginal estrogens 1, 4
• Methenamine hippurate (1g twice daily) 4
• Lactobacillus-containing probiotics 4

Antibiotic Prophylaxis (When Necessary)

• Post-coital prophylaxis for UTIs associated with sexual activity:
  • Nitrofurantoin 50-100 mg
  • Trimethoprim-sulfamethoxazole 40/200 mg
  • Trimethoprim 100 mg 4
• Should be approached judiciously due to risk of antibiotic resistance 1

Diagnostic Approach

• Urine culture typically reveals >100,000 organisms per milliliter 1
• Obtain urine culture before starting antibiotics to guide therapy 4
• Consider imaging if symptoms persist or in complicated cases 4
• Imaging is generally low yield in patients with uncomplicated recurrent UTIs 1

Common Pitfalls and Caveats

• Treating asymptomatic bacteriuria unnecessarily (common in older women) 4
• Failing to identify underlying anatomical abnormalities in complicated cases
• Overuse of antibiotics leading to resistance development 4, 6
• Not considering intracellular bacterial communities as a source of recurrence 2
• Neglecting to address modifiable risk factors before initiating antibiotic prophylaxis 1