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Causes of urinary tract infections

Urine does not normally contain microorganisms, but if urine is obstructed from leaving the body or retained in the bladder, it provides a good environment for bacteria to grow. Most urinary tract infections are due to bacteria that are introduced into the opening of the urethra. They stick to the walls of the urethra, multiplying and moving up the urethra to the bladder.

Most UTIs remain in the lower urinary tract (urethra or bladder) where they cause annoying symptoms, such as a burning sensation during urination, but are more easily treated. While these infections are easily treated in most cases, if inadequately treated, the infection may spread up through the ureters, and into the kidneys which is a more serious condition which in rare cases can cause permanent kidney damage  and needs a longer course of antibiotics. In some cases a urinary tract infection may lead to an infection in the bloodstream (septicaemia) that can be life-threatening.

Although a variety of bacteria can cause UTIs, most (80 to 90 percent) are due to Escherichia coli, a bacterium that is common in the gastrointestinal tract and is routinely found in stool (faeces). Other bacteria that may cause UTIs include species of: Proteus, Klebsiella, Enterococcus and Staphylococcus. Occasionally, a UTI may be due to a yeast, such as Candida albicans. Urethritis (infection involving the urethra) is often due to a sexually transmitted disease such as herpes, chlamydia, or gonorrhoea.

Although anyone at any age can have a UTI, women are much more likely than men to have them. It is thought that this is partly anatomical, because a woman’s urethra is shorter (less distance for bacteria to travel) and because their anus and vagina are relatively close to the urethra. Anything that slows down the passage of the urine, blocks it, or introduces bacteria into the urinary tract can raise a patient’s risk of having a UTI.

Conditions that cause an increased risk for developing a UTI include:

  • Anatomical problems (such as narrowing of the urethra or ureters)
  • Urine retention (the bladder does not empty completely)
  • Vesicoureteral reflux (the abnormal flow of urine from the bladder back through the ureters
  • Kidney stones
  • Bladder catheterisation (especially long term)
  • Spinal cord injuries (leads to the inability of normal bladder emptying)
  • Diabetes (it causes changes to the immune system, damage to the kidneys and often results in sugar in the urine – promoting the growth of bacteria)
  • Any condition that suppresses the immune system
  • In men, an enlarged prostate may inhibit the flow of urine.

Last Review Date: November 13, 2019