Escherichia coli (E. coli) causes about 80% of urinary tract infections (UTIs) in adults. These bacteria are normally present in the colon and may enter the urethral opening from the skin around the anus and genitals. Women may be more susceptible to UTI because their urethral opening is near the source of bacteria (e.g., anus, vagina) and their urethra is shorter, providing bacteria easier access to the bladder.
Other bacteria that cause urinary tract infections include Staphylococcus saprophyticus (5 to 15% of cases), Chlamydia trachomatis, and Mycoplasma hominis. Men and women infected with chlamydia trachomatis or mycoplasma hominis can transmit the bacteria to their partner during sexual intercourse, causing UTI.
Sexual intercourse triggers UTI in some women, for unknown reasons. Women who use a diaphragm develop infections more often, and condoms with spermicidal foam may cause the growth of E. coli in the vagina, which may enter the urethra.
Urinary catheterization (i.e., insertion of a small tube into the bladder through the urethra to drain urine) can also cause UTI by introducing bacteria into the urinary tract. The risk for developing a UTI increases when long-term catheterization is required.
In infants, bacteria from soiled diapers can enter the urethra and cause UTI. E. coli may also enter the urethral opening when young girls do not wipe from front to back after a bowel movement.