Normally, a UTI is prevented by acidic urine, complete bladder emptying, a competent ureterovesical junction to prevent urine backflow, and bacteriostatic properties of the urethra and bladder. Ordinarily, the urinary tract above the urethra is sterile; therefore, if bacteria ascends the urethra and colonizes the bladder, an inflammatory process is initiated. The process of voiding routinely cleanses the bladder and urethra. Thus, anything that interferes with this process, such as outflow obstruction, postponement of voiding, pregnancy, or diabetes mellitus, increases the risk of a UTI. Use of a spermicide or diaphragms (both alter normal vaginal bacterial flora), fecal incontinence, and catheterization have also been implicated as UTI risk factors. Approximately 1% of adults who have a straight catheterization and almost 100% of patients who have an indwelling catheter develop a UTI within 3 to 4 days of catheter placement.

Bacteria are introduced into the bladder during the catheterization procedure. The process of catheterization also causes irritation and
minute scraping of the urethra, creating a portal for bacterial entry. Additionally, an indwelling catheter prevents normal flushing of the urethra by urine, and bacteria may travel to the bladder through the catheter itself or via the exudate that collects between the outside of the catheter and the urethral walls. As bacteria adhere to the catheter, they produce a protective film that covers the surface of the catheter and protects the bacteria against antibiotic action, making eradication of the infection difficult.