23.2 Bacterial infections of the urinary system

Learning objectives

• Identify the most common bacterial pathogens that can cause urinary tract infections
• Compare the major characteristics of specific bacterial diseases affecting the urinary tract

Urinary tract infections (UTIs) include infections of the urethra, bladder, and kidneys, and are common causes of urethritis, cystitis, pyelonephritis, and glomerulonephritis. Bacteria are the most common causes of UTIs, especially in the urethra and bladder.

Cystitis

Cystitis is most often caused by a bacterial infection of the bladder, but it can also occur as a reaction to certain treatments or irritants such as radiation treatment, hygiene sprays, or spermicides. Common symptoms of cystitis include dysuria (urination accompanied by burning, discomfort, or pain), pyuria (pus in the urine), hematuria (blood in the urine), and bladder pain.

In women, bladder infections are more common because the urethra is short and located in close proximity to the anus, which can result in infections of the urinary tract by fecal bacteria. Bladder infections are also more common in the elderly because the bladder may not empty fully, causing urine to pool; the elderly may also have weaker immune systems that make them more vulnerable to infection. Conditions such as prostatitis in men or kidney stones in both men and women can impact proper drainage of urine and increase risk of bladder infections. Catheterization can also increase the risk of bladder infection (see Case in Point: Cystitis in the Elderly ).

Gram-negative bacteria such as Escherichia coli (most commonly), Proteus vulgaris , Pseudomonas aeruginosa , and Klebsiella pneumoniae cause most bladder infections. Gram-positive pathogens associated with cystitis include the coagulase-negative Staphylococcus saprophyticus , Enterococcus faecalis , and Streptococcus agalactiae . Routine manual urinalysis using a urine dipstick or test strip can be used for rapid screening of infection. These test strips ( [link] ) are either held in a urine stream or dipped in a sample of urine to test for the presence of nitrites, leukocyte esterase , protein, or blood that can indicate an active bacterial infection. The presence of nitrite may indicate the presence of E. coli or K. pneumonia; these bacteria produce nitrate reductase , which converts nitrate to nitrite. The leukocyte esterase (LE) test detects the presence of neutrophils as an indication of active infection.

Low specificity, sensitivity, or both, associated with these rapid screening tests require that care be taken in interpretation of results and in their use in diagnosis of urinary tract infections. Therefore, positive LE or nitrite results are followed by a urine culture to confirm a bladder infection. Urine culture is generally accomplished using blood agar and MacConkey agar, and it is important to culture a clean catch of urine to minimize contamination with normal microbiota of the penis and vagina. A clean catch of urine is accomplished by first washing the labia and urethral opening of female patients or the penis of male patients. The patient then releases a small amount of urine into the toilet bowl before stopping the flow of urine. Finally, the patient resumes urination, this time filling the container used to collect the specimen.