10.2: Pathogenesis

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The urinary tract is normally sterile above the level of distal urethra. In the majority of UTIs, the organism ascends to the urinary tract via the ascending route. The female urethra is short and close to the faecal reservoir (source of most organisms causing UTIs) hence the higher incidence of UTIs in females compared to males. The main defence mechanism against infection is the hydrokinetic effect of regular effective voiding. For a significant infection to develop, an organism needs to gain access to the urinary tract, adhere to the urothelium and multiply in numbers. Virulent bacteria can overcome normal host defence mechanisms whereas less virulent bacteria result in significant infections in patients with abnormal urinary tracts or those with compromised immunity. During sexual intercourse the faecal organisms causing UTIs colonise the vagina and thus sexual activity encourages ascending infection.

The microbiology of UTI is as follows:

Escherichia coli (E coli) responsible for:
- 85% of community acquired } UTIs
- 50% of hospital acquired }
other common Gram-negative organisms:
- Klebsiella
- Proteus
- Pseudomonas
Gram-positive organisms can also cause UTIs:
- Strep. faecalis
- Staph. aureus
- Staph. epidermidis
- Staph. saprophyticus
In diabetics and immunocompromised patients fungi (Candida) and viruses (adenovirus, cytomegalovirus) cause a significant proportion of UTIs