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 after bacteria overcome the system's strong natural defenses. In spite of these defenses, UTIs are the most common of all 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 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 into the bladder, which rests on top of the pelvic floor. This 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, which is then eliminated from the body via another tube called the urethra, the lowest part of the urinary tract. (In men it 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 are structurally designed to prevent urine from backing up into the kidney.
• The prostate gland in men secretes infection-fighting substances.
• The immune system in both sexes continuously fights bacteria and other harmful micro-invaders. In addition, immune system defenses and antibacterial substances in the mucous lining of the bladder eliminate many organisms.
• In normal fertile women, the vagina is colonized by lactobacilli, beneficial microorganisms that maintain a highly acidic environment (low pH). Acid 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