What causes a UTI?

The most common causes of UTI infections (about 80%) are Escherichia coli bacterial strains that usually inhabit the colon. However, many other bacteria can occasionally cause an infection (for example, Klebsiella, Pseudomonas, Enterobacter, Proteus, Staphylococcus, Mycoplasma, Chlamydia, Serratia and Neisseria spp) but are far less frequent causes than E. coli. In addition, fungi (Candida and Cryptococcus spp) and some parasites (Trichomonas, Schistosoma) also may cause UTIs; Schistosoma causes other problems, with bladder infections as only a part of its complicated infectious process. In the U.S., most infections are due to Gram-negative bacteria with E. coli causing the majority of infections.

What are UTI risk factors?

There are many risk factors for UTIs. In general, any interruption or impedance of the usual flow of urine (about 50 cc per hour in normal adults) is a risk factor for a UTI. For example, kidney stones, urethral strictures, enlarged prostate, or any anatomical abnormalities in the urinary tract increases infection risk. This is due in part to the flushing or wash-out effect of flowing urine; in effect the pathogens have to "go against flow" because the majority of pathogens enter through the urethra and have to go retrograde (against a barrier, urine flow) to reach the bladder, ureters, and eventually the kidneys. Many investigators suggest that women are far more susceptible than men to UTIs because their urethra is short and its exit (or entry for pathogens) is close to the anus and vagina, which can be sources for pathogens.
People who require catheters have an increased risk (about 30% of patients with indwelling catheters get UTIs) as the catheter has none of the protective immune systems to eliminate bacteria and offers a direct connection to the bladder.
There are reports that suggest that women who use a diaphragm or who have partners that use condoms with spermicidal foam are at increased risk for UTIs. In addition, females who become sexually active seem to have a higher risk of UTI; some investigators term these UTIs as "honeymoon cystitis."
Men over 60 have a higher risk for UTIs because many men at or above that age develop enlarged prostates that may cause slow and incomplete bladder emptying.
Occasionally, people with bacteremia (bacteria in the bloodstream) have the infecting bacteria lodge in the kidney; this is termed hematogenous spread. Similarly, people with infected areas that are connected to the urinary tract (for example, prostate, epididymis, or fistulas) are more likely to get a UTI. Additionally, patients who undergo urologic surgery also have and increased risk of UTIs. Pregnancy does not apparently increase the risk of UTIs according to some clinicians; others think there is an increased risk between weeks six through 26 of the pregnancy. However, most agree that if UTIs occur in pregnancy, the risk of the UTI progressing in seriousness to pyelonephritis is increased according to several investigators. In addition, their baby may be premature and have a low birth weight. Patients with chronic diseases such as diabetics or those who are immunosuppressed (HIV or cancer patients) also are at higher risk for UTIs.