Etiology and pathogenesis

Urine is an excellent culture medium and contains sufficient nutrients to support bacterial growth to a density of 107CFUml '. Ideal conditions, of urine pH 4-7 and concentration of 300-800 mosmol kg \ are common. Bacteria most commonly enter via the urethra (ascending infection) or may enter via the bloodstream. Ascending infections, which account for the great majority of cystitis and pyelonephritis, are mainly due to organisms of the normal bowel flora - principally Escherichia coli (75% or more). Predisposing factors are shown in Table 1. Bladder

Table 1 Predisposing factors for urinary tract infection

Cystitis Pyelonephritis

Table 1 Predisposing factors for urinary tract infection

Cystitis Pyelonephritis

Urinary stasis

Normal kidney

Outflow obstruction

Pregnancy

Impaired bladder contraction

Stones

Reflux into ureter or

Diabetes

diverticulum

Previous episodes

Foreign body

Abnormal kidney

Bladder stone

Scars (from previous

Bladder tumor

reflux/infection)

Instrumentation

Obstruction

Catheter

Duplex kidney

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