Etiology

• Infection ascends from the urethra to the bladder.

• Can ascend further to the ureters and the renal pelvises (see “Pyelonephritis”)

• Causative organisms

  • Escherichia coli: leading cause of UTI (approx. 80%) [1]

  • Staphylococcus saprophyticus: 2nd leading cause of UTI in sexually active women

  • Klebsiella pneumoniae: 3rd leading cause of UTI

  • Proteus mirabilis

    • Produces ammonia, giving the urine a pungent or irritating smell

    • Associated with struvite stone formation

  • Nosocomial bacteria: Serratia marcescens, Enterococci spp., and Pseudomonas aeruginosa are associated with increased drug resistance.

  • Enterobacter species

  • Ureaplasma urealyticum

Viruses

• Immunocompromised patients and children are particularly susceptible to viral UTIs. [2]

• Adenovirus, cytomegalovirus, and BK virus are commonly involved in hemorrhagic cystitis. [3]

Fungi [4]

• Yeast: rare (usually Candida species) [5]

• Disseminated fungal infections (e.g., Blastomyces dermatitidis, Cryptococcus neoformans)

• Structural or functional abnormalities of the urinary tract [6]

  • Prevent bladder emptying and/or result in urinary stasis

  • Examples include:

    • Benign prostatic hyperplasia

    • Congenital malformations causing vesicoureteral reflux

    • Urinary bladder diverticulum

    • Neurogenic bladder

    • Urinary tract calculi

• Sex

  • Female individuals: anatomically predisposed because the urethra is shorter and anal and genital regions are in close proximity → bacteria spreading from the anal region → colonization of vagina → ascending UTIs [7]

  • Male individuals: higher risk in uncircumcised male infants [8]

• Pregnancy: hormonal changes during pregnancy → urinary stasis and vesicoureteral reflux → increased risk of UTIs

• Postmenopause: ↓ estrogen → ↓ vaginal lactobacilli → ↑ vaginal pH → ↑ colonization by E. coli [7]

• Chronic constipation: common cause of UTIs in children

• Prior conditions

  • Previous UTI [7]

  • History of kidney surgery

  • Immunosuppression

  • Diabetes mellitus

• Medication: recent use of antibiotics

• Sexual intercourse

  • Postcoital cystitis (honeymoon cystitis): a lower urinary tract infection that occurs in women after recent sexual activity, which can cause irritation of the urethral meatus and facilitate bacterial entry into the urethra (e.g., from the genital and/or anal region).

  • Diaphragm and spermicide use

• Catheter-associated urinary tract infection (CAUTI)

  • Caused by indwelling urinary catheters

  • Most common cause of nosocomial urinary tract infection