Urinary tract infection

are recognised frequently in general practice and are the commonest type of
hospital-acquired infection. Infections occur in both sexes and at all ages, but are
commonest in females and at the extremes of age. Whilst many infections are
mild, renal infections may lead to long term renal damage, and the urinary tract is a
common source of life threatening Gram-negative bacteraemia. Several clinical
syndromes are recognised (Table 1), the commonest being lower UTI of the bladder
(cystitis). Upper UTI (pyelonephritis) may result from haematogenous or ascending
routes of infection.
Table 1.
Pathogenesis
The normal urinary tract is protected from infection by a variety of defence
mechanisms. Foremost in these is the regular flushing during voiding which
removes organisms from the distal urethra. Between voiding such organisms may
ascend the urethra and hence, infection is commoner in females because the
urethra is comparatively short. Other defence factors include antibacterial
secretions into the urine and urethra. The pathogenesis of UTIs is summarised in
Table 2. The commonest pathogens are Gram-negative rods particularly
enterobactericeae (‘coliforms’, especially Esch. coli). Coagulase-negative
staphylococci cause infections in young women and in hospitalised patients (Staph.
saprophyticus). The latter group also has a wider range of pathogens isolated,
including other Gram-negatives such as Pseudomonas aeruginosa, mainly as a
result of greater antibiotic selective pressure.
Host
Factors
Shorter urethra More infections in females
Obstruction Enlarged prostate, pregnancy, stones,
tumours
Neurological Incomplete emptying, residual urine
Ureteric reflux Ascending infection from bladder especially
in children
Bacterial
Factors
Faecal flora Potential urinary pathogens colonise
periurethral area
Adhesion Fimbriae and adhesins allow attachment to
urethral and bladder epithelium
K antigens Allow some E. coli to resist host defences by
Lower urinary
Tract
Bacterial cystitis Frequency and dysuria often with
pyuria and haematuria
Abacterial cystitis As above but without ‘significant
bacteriuria’
Prostatitis Fever, dysuria, frequency with
perineal and low back pain
Upper urinary
Tract
Acute
pyelonephritis
Symptoms of cystitis plus fever and
loin pain
Chronic
interstitial
nephritis
Renal impairment following chronic
inflammation – infection one of
many causes
Asymptomatic Covert
bacteriuria
Detected only by culture important
in children and pregnancy