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Bacteria and other microorganisms cause UTIs
Bacteria cause most urinary tract infections. The most common cause is Escherichia coli (E. coli), which live in the intestines of healthy people and don't cause infections there.
E. coli have small hair-like projections that help them stick tightly to cells in the walls of the urinary tract. A coat on their outside surface protects E. coli against the body's natural immune system, which tries to kill them. Other kinds of bacteria and some yeasts can cause urinary tract infections, too.
How do bacteria get into the urinary tract?
Normally, the inside of the urinary tract has no bacteria. The bacteria have to enter from the outside to cause an infection.
They come from nearby
The most likely place for bacteria to come from is the lower end of the intestine. E. coli are bacteria that normally live in the intestinal tract without causing disease. In women, the opening of the urinary tract (the urethra) is close to the opening of the intestine (the anus). This is one reason women are 50 times more likely to get urinary tract infections than men.
The bacteria that normally live in the vagina of a healthy woman are believed to provide some protection against disease-causing bacteria by keeping them from attaching themselves to the cells near the urethral opening. So it is not a good idea to try to get rid of these "good" bacteria by methods like douching.
Bacteria can enter the urinary tract during sex. Sexual activity can move bacteria from a man's penis or a woman's skin into the opening where urine comes out of her urethra. Men who have anal sex without using a condom (unprotected anal sex) may increase their chances of getting urinary tract infections.
Using a urinary catheter can lead to urinary tract infection. The bacteria may get in when the catheter is inserted through the urethra to the bladder, or gradually move up the catheter walls and into the bladder.
Instruments that health care professionals use to check the health of the urethra and bladder sometimes carry bacteria into the bladder.
Why aren't all the bacteria washed out of the urinary tract?
The flow of urine out of the body usually washes bacteria out as well. But the kinds of bacteria that infect the urinary tract can attach themselves tightly. These bacteria continue to cling during urination and are not washed out.
Some people can't completely empty their bladder when they urinate. As a result, some bacteria remain. Bacteria that stay in the bladder for a long time can multiply.
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