Pathophysiology

Urinary tract infections that are caused by bacteria of faecal origin (the commonest cause) involve either the bladder, the upper urinary tract (ureter, pelvicalyceal system, kidney) or both.

Some facts about UTIs:

• the bladder is most commonly affected
• females are more commonly affected than men
• at least 50% of women suffer a UTI sometime in their life

The pathogens may reach the urinary tract

• via haematogenous or lymphatic spread
• as an ascending infection from the urethra
• during bladder catheterization or instrumentation (1).

In females the likely route of infection is via the urethra (which is only 3 cm long). Probably the organisms are spread from perineal skin, particularly during intercourse.

Usually the bladder is flushed by the frequent passage of newly produced urine (2). However if for some reason there is bladder stasis (e.g. immobility, dehydration) then the bacteria can multiply.

Faecal organisms can also enter the bladder via spread from the bloodstream. This particularly true of elderly or immobilised patients. Urinary tract infections develop when the balance between host defence mechanisms and the virulence of the invading organism is distorted.(3)

Reference:

1. European Association of Urology 2009. Guidelines on urological infections
2. Health Protection Agency (HPA) 2009. National standard method. Investigation of urine
3. Abraham SN, Miao Y. The nature of immune responses to urinary tract infections. Nat Rev Immunol. 2015 Oct;15(10):655-63.