Introduction:

A urinary tract infection (UTI) is a condition where one or more parts of the urinary system (the kidneys, ureters, bladder, and urethra) become infected. UTIs are the most common of all bacterial infections and can occur at any time in the life of an individual. Almost 95% of cases of UTIs are caused by bacteria that typically multiply at the opening of the urethra and travel up to the bladder. Much less often, bacteria spread to the kidney from the bloodstream.

The male and female urinary tracts are relatively the same except for the length of the urethra.

The Urinary System. The urinary system helps maintain proper water and salt balance throughout the body and also expels urine from the body. It is made up of the following organs and structures:

• The two kidneys, located on each side below the ribs and toward the middle of the back, play the major role in this process. They filter waste products, water, and salts from the blood to form urine.
• Urine passes from each kidney to the bladder through thin tubes called ureters.
• Ureters empty the urine into the bladder, which rests on top of the pelvic floor. The pelvic floor is a muscular structure similar to a sling running between the pubic bone in front to the base of the spine.
• The bladder stores the urine. When the bladder becomes filled, the muscles in the wall of the bladder squeeze, and the urine leaves the body via another tube called the urethra. In men the urethra is enclosed in the penis. In women, it leads directly out.)

Defense Systems Against Bacteria. Infection does not always occur when bacteria are introduced into the bladder. A number of defense systems protect the urinary tract against infection-causing bacteria:

• Urine itself functions as an antiseptic, washing potentially harmful bacteria out of the body during normal urination. (Urine is normally sterile, that is, free of bacteria, viruses, and fungi.)
• The ureters join into the bladder in a manner designed to prevent urine from backing up into the kidney when the bladder squeezes urine out through the urethra.
• The prostate gland in men secretes infection-fighting substances.
• The immune system defenses and antibacterial substances in the mucous lining of the bladder eliminate many organisms.
• In healthy women, the vagina is colonized by lactobacilli, beneficial microorganisms that maintain a highly acidic environment (low pH) that is hostile to other bacteria. Lactobacilli also produce hydrogen peroxide, which helps eliminate bacteria and reduces the ability of Escherichia coli (E. coli) to adhere to vaginal cells. (E. coli is the major bacterial culprit in urinary tract infections.)

Types of UTIs. UTIs are generally classified as:

• Uncomplicated or complicated, depending on the factors that trigger the infections
• Primary or recurrent, depending on whether the infection is occurring for the first time or is a repeat event

Uncomplicated Urinary Tract Infections (UTIs)

Uncomplicated UTIs are due to a bacterial infection, most often E. coli. They affect women much more often than men.

Cystitis. Cystitis, or bladder infection, is the most common urinary tract infection. It occurs in the lower urinary tract (the bladder and urethra) and nearly always in women. In most cases, the infection is brief and acute and only the surface of the bladder is infected. Deeper layers of the bladder may be harmed if the infection becomes persistent, or chronic, or if the urinary tract is structurally abnormal.

Pyelonephritis (Kidney Infection). Sometimes the infection spreads to the upper tract (the ureters and kidneys). This is called pyelonephritis, or more commonly, a kidney infection.

Complicated Urinary Tract Infections

Complicated infections, which occur in men and women of any age, are also caused by bacteria but they tend to be more severe, more difficult to treat, and recurrent. They are often the result of:

• Some anatomical or structural abnormality that impairs the ability of the urinary tract to clear out urine and therefore bacteria.
• Catheter use in the hospital setting or chronic indwelling catheter in the outpatient setting,
• Bladder and kidney dysfunction, or kidney transplant (especially in the first 3 months after transplant).

Recurrences occur in up to 50 - 60% of patients with complicated UTI if the underlying structural or anatomical abnormalities are not corrected.

Recurrent Urinary Tract Infections

Most women who have had an uncomplicated UTI have occasional recurrences. About 25 - 50% of these women can expect another infection within a year of the previous one. Between 3 - 5% of women have ongoing, recurrent urinary tract infections, which follow the resolution of a previous treated or untreated episode.

Recurrence is often categorized as either reinfection or relapse:

• Reinfection. About 80% of recurring UTIs are reinfections. A reinfection occurs several weeks after antibiotic treatment has cleared up the initial episode and can be caused by the same bacterial strain that caused the original episode or a different one. The infecting organism is usually introduced through fecal bacteria and moves up through the urinary tract.
• Relapse. Relapse is the less common form of recurrent urinary tract infection. It is diagnosed when a UTI recurs within 2 weeks of treatment of the first episode and is due to treatment failure. Relapse usually occurs in kidney infection (pyelonephritis) or is associated with obstructions such as kidney stones, structural abnormalities or, in men, chronic prostatitis.

Asymptomatic Urinary Tract Infection (Asymptomatic Bacteriuria)

When a person has no symptoms of infection but significant numbers of bacteria have colonized the urinary tract, the condition is called asymptomatic UTI (also called asymptomatic bacteriuria). The condition is harmless in most people and rarely persists, although it does increase the risk for developing symptomatic UTIs.

Screening for asymptomatic bacteriuria is not necessary during most routine medical examinations, with the following exceptions:

• Pregnant women. Pregnant women with asymptomatic bacteriuria have an increased risk of acute pyelonephritis in their second or third trimester. Therefore, they need screening and treatment for this condition. Guidelines recommend that pregnant women be screened for asymptomatic bacteriuria at 12 - 16 weeks gestation or at the first pre-natal visit, if later.
• People undergoing urologic surgery (such as prostate surgery in men). The presence of an infection during surgery can lead to serious consequences.