A urinary tract infection (UTI) is a condition where one or more structures in the urinary tract become infected after bacteria overcome its 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 (known as the ascending route). Much less often, bacteria spread to the kidney from the bloodstream.

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

Different classifications have been devised to help doctors choose treatments and determine the causes of UTIs.

Primary or Recurrent UTIs. UTIs are classified as primary or recurrent, depending on whether they are the first infection or whether they are repeat events.

Community- or Hospital-Acquired. UTIs are also sometimes grouped according to where they are acquired:

  • Community-Acquired Infections. Most UTIs are thought to develop in the community at large. It is unclear how primary community-acquired infections occur or how they are spread. Although most cases have been thought to arise sporadically, a rare outbreak caused by drug-resistant bacteria in three states between 1996 and 2000 suggests epidemic spread of community-acquired infections could be more common than previously thought and may be spread via contaminated food. Most community-acquired infections are not serious and probably develop when the intestines become colonized with bacteria that are also predisposed to infecting the urinary tract.
  • Hospital-Acquired Infections. UTIs are also commonly acquired in the hospital, often due to contaminated urinary catheters. Hospital-acquired infections (known as nosocomial infections) tend to be more serious because the bacteria that cause them are often resistant to drug treatment and patients are often in poor general health.

Uncomplicated and Complicated. UTIs are also sometimes further defined as either being uncomplicated or complicated depending on the factors that trigger the infections.

  • Uncomplicated infections are only associated with bacterial infection, most often Escherichia coli (E. coli). They affect women much more often than men.
  • Complicated infections, which occur nearly as often in men as women, are also caused by bacteria but they occur as a result of some anatomical or structural abnormality. Often they are associated with catheter use in the hospital setting, bladder and kidney dysfunction, or kidney transplant (especially in the first three 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.

Classifications Based on Symptoms and Levels of Infection. UTIs can also occur without symptoms and with symptoms but very low bacterial levels.

  • When bacteria are present and there are no symptoms it is called asymptomatic UTI or also bacteriuria.
  • Some patients can also have symptoms of infection with very low bacterial counts. In such cases, the condition is called acute urethral syndrome.

Uncomplicated Urinary Tract Infections (UTIs)

Cystitis. Cystitis is the most common urinary tract infection and is sometimes referred to as acute uncomplicated UTI. 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). When infection spreads to the upper tract (the ureters and kidneys) it is called pyelonephritis, or more commonly, kidney infection. As many as half of all women with cystitis may have infections of the upper urinary tract at the same time as cystitis.

Kidney anatomy picture

Click the icon to see an image of the kidney.

Urethritis. When infection is limited only to the urethra, the infection is known as urethritis. This is a common sexually transmitted disease in men.

Complicated Urinary Tract Infections

Complicated UTIs may develop because of any one of a number of physical problems and affect any gender and age group. The common feature in most complicated UTIs is the inability of the urinary tract to clear out bacteria because of a physical condition that causes obstruction to the flow of urine or problems that hinder treatment success.

Recurrent Urinary Tract Infections

Most women who have had an uncomplicated UTI have occasional recurrences. A 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 the rectal region from fecal matter 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). (In general, there must be at least 100,000 bacteria per milliliter of urine.) 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 a 30% risk for acute pyelonephritis in their second or third trimester. Therefore, they need screening and treatment for this condition.
  • People undergoing urologic surgery (such as prostate surgery in men). The presence of an infection during surgery can lead to serious consequences.

Some groups recommend screening women with diabetes for asymptomatic bacteriuria. However, a 2003 study suggested that treating women who test positive for this condition does not reduce their risk of complications from UTIs. Asymptomatic bacteriuria may be an indicator for serious health problems in the elderly, but screening for the condition is not warranted in this group.

Acute Urethral Syndrome

Some people have symptoms of cystitis but have a bacterial count lower than that ordinarily found in UTI. Such patients are sometimes diagnosed with acute urethral syndrome. This condition is usually caused by E. coli or other bacteria that cause cystitis, but in lower numbers, or by a sexually transmitted disease such as Chlamydia or gonorrhea.

Interstitial Cystitis

Interstitial cystitis (IC) is an inflammation of the bladder wall that occurs almost exclusively in women. The average age of patients with IC is 40 years, but 25% of cases occur in women under age 30. Symptoms are very similar to cystitis, but no bacteria are present. These women often complain of experiencing pain during sex. Pelvic pain, depression, and stress may intensify symptoms. Women with IC also frequently suffer from other conditions, including allergies, urinary incontinence, sinusitis, and irritable bowel syndrome (IBS). Some doctors think that IC may be related to autoimmune diseases such as fibromyalgia and lupus.

IC is difficult to diagnose and treat. Pentosan (Elmiron) is the most frequent drug treatment, but doctors prescribe other medications as well (see Medications section). Some evidence suggests that diet can worsen IC symptoms. According to research presented at the 2006 American Urological Association scientific meeting, patients should avoid coffee (both caffeinated and decaf), alcohol, cola, vinegar, citrus fruits, tomatoes, chili, strawberries, pineapple, onions, pizza, chocolate, and apples

The Urinary System and Its Defenses

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-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, which is 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 E. coli to adhere to vaginal cells. (E. coli is the major bacterial culprit in urinary tract infections.)
  • Some interesting research suggests that when bacteria infect the bladder, the cells that line the bladder literally sacrifice themselves and self-destruct (a process called apoptosis). In so doing, they fall away from the lining, carrying the bacteria with them. This eliminates about 90% of the E. coli.
  • Some researchers have identified a possible natural antibiotic called human beta-defensin-1 (HBD-1), which fights E-coli within the female urinary and reproductive tracts.
Prostate picture

Click the icon to see an image of the prostate gland.