Urinary tract infection (uti)

• E. coli

• renal system, human

• Pseudomonas aeruginosa

• E. coli: E. coli bacteria in laboratory culture

Urinary tract infection (UTI),  in humans, inflammation of the renal system characterized by frequent and painful urination and caused by the invasion of microorganisms, usually bacteria, into the urethra and bladder. Infection of the urinary tract can result in either minor or major illness. For example, an attack of cystitis—inflammation of the bladder—may cause only a small amount of pain and discomfort, whereas infection that spreads into the upper urinary tract may lead to acute complications, such as obstruction of the ureter and kidney failure, or to chronic conditions, such as incontinence or kidney scarring that gradually progresses to kidney failure. Severe or recurrent UTI can result in lifelong discomfort and decrease in quality of life.

Risk factors

UTIs are very common and can occur in people of all ages. However, women are affected about 30 times more often than men; roughly one in five women experiences a UTI in her lifetime. Girls and women are at high risk of infection because of the short female urethra. In addition, sexual intercourse, especially when a diaphragm is used for contraception, and pregnancy, when there may be partial stagnation of the urine from pressure on the urinary tract, significantly increase the susceptibility of women to UTI. Many women experience recurrent UTIs, and those who have had three or more infections are likely to have frequent recurrences throughout their lifetimes. It is unclear why some women are at high risk for repeated infection. There is evidence that certain antigens of the P blood group system that are expressed on the surfaces of cells lining the urinary tract act as adhesion sites for bacteria, thereby facilitating infection. Postmenopausal women may have recurrent UTIs because decreasing levels of estrogen cause thinning of the vaginal epithelium, thereby reducing defense against invasion by microorganisms.

Other populations at risk of infection include men over age 50, in whom onset of prostatic disease may lead to urinary infection. Infants also are at risk, since diapers can facilitate the entry of organisms into the urethra. In addition, people affected by kidney stones, diabetes, disorders of the immune system, and abnormalities of the renal system are at increased risk of infection. In some patients, the introduction of a catheter into the bladder may be necessary to relieve urethral obstruction; however, this procedure increases risk of UTI.

Causes

The most common cause of UTI is infection with Escherichia coli, a type of bacterium that normally inhabits the bowel, where it is relatively harmless. These organisms become a cause of UTI only when they enter the urethra. The second most common bacterial cause of UTI is Staphylococcus saprophyticus, which normally occurs on the skin of some humans. Bacteria that are rare causes of UTIs but that may be involved in severe infections include Proteus mirabilis and organisms in the genera Klebsiella, Mycoplasma, Enterococcus, Pseudomonas, and Serratia. In rare cases, fungal organisms, such as Candida and Coccidioides, may be involved in complicated UTIs, which involve infection with multiple, different organisms. In addition, threadworms, flukes, and other parasites, as well as viruses such as HSV-2 (herpes simplex virus type 2) have been identified as infrequent causes of UTIs. Bacteria that cause UTIs are almost always transmitted during sexual intercourse, and thus proper hygiene of both partners is a useful way to prevent infection.