Urinary Tract Infection - Causes

Causes

The bacterial strains that cause UTIs include:

• Escherichia (E.) coli is responsible for most uncomplicated cystitis cases in women, especially in younger women. E. coli is generally a harmless microorganism originating in the intestines. If it spreads to the vaginal opening, it may invade and colonize the bladder, causing an infection. The spread of E. coli to the vaginal opening most commonly occurs when women or girls wipe themselves from back to front after urinating, or after sexual activity.
• Staphylococcus saprophyticus accounts for 5 - 15% of UTIs, mostly in younger women.
• Klebsiella, Enterococci bacteria, and Proteus mirabilis account for most of remaining bacterial organisms that cause UTIs. They are generally found in UTIs in older women.
• Rare bacterial causes of UTIs include ureaplasma urealyticum and Mycoplasma hominis, which are typically harmless organisms.

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Organisms in Severe or Complicated Infections

• The bacteria that cause kidney infections (pyelonephritis) are generally the same bacteria that cause cystitis. There is some evidence, however, the E. coli strains in pyelonephritis are more virulent (able to spread and cause illness).
• Complicated UTIs that are related to physical or structural conditions are apt to be caused by a wider range of organism. E. coli is still the most common organism, but others include Klebsiella, P. mirabilis, and Citrobacter.
• Fungal organisms, such as Candida species. (Candida albicans causes the "yeast infections" that also occur in the mouth, digestive tract, and vagina.)
• Other bacteria associated with complicated or severe infection include Pseudomonas aeruginosa, Enterobacter, and Serratia species gram-positive organisms (including Enterococcus species).

Bacterial Strains in Recurrent UTIs

Recurring infections are often caused by different bacteria than those that caused a previous or first infection.

Factors in Overcoming the Bacterial Defense Systems

Changes in the amount or type of acid within the genital and urinary tracts are contributors to lowering the resistance to infection. For example, beneficial organisms called lactobacilli increase the acidic environment in the female urinary tract. Reductions in their number (which, for example, occur with estrogen loss after menopause), increase pH and therefore the risk of infection.

Review Date: 06/17/2010
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org).