What is being tested?The urine culture test detects and identifies
bacteria and yeast in the urine. Urine is produced by the kidneys, located on either side of the spine at the base of the ribcage. The kidneys filter waste out of the blood and produce urine, a yellow fluid, to carry the wastes out of the body. Urine travels through tubes called ureters from the kidneys to the bladder, where it is stored temporarily, and then through the urethra as it is voided. Urine is generally sterile, but sometimes bacteria (or, more rarely, yeast) can move from the skin outside the urethra and migrate back up the urinary tract to cause a
urinary tract infection (UTI).
With a urine culture, a small sample of urine is placed on one or more agar plates (a thin layer of a nutrient gel) and incubated at body temperature. Any microorganisms that are present in the urine sample grow over the next 24 to 48 hours as small circular colonies. The size, shape, and color of these colonies give clues as to which bacteria are present, and the number of colonies indicates the quantity of bacteria originally present in the urine sample. A laboratorian observes the colonies on the agar plate, counting the total number and determining how many types have grown. Ideally, if a good clean sample was collected for the test (see below), then the only bacteria present should be due to an infection. Usually, this will be a single type of bacteria that will be present in relatively large numbers. Sometimes, more than one type of bacteria will be present. This may be due to an infection that involves more than one pathogen (disease-causing microorganism); however, it is more likely to be due to contamination from the skin picked up during the urine collection.
The laboratorian will take a colony from each type of bacteria present that appears significant in number or type, smear it on a slide, dry it, and stain it with dyes (called a gram stain). The laboratorian then examines the microorganisms under the microscope. Different types of bacteria will have different colors and shapes that can differentiate different types of bacteria present. For instance, the bacterium Eschericia coli, which causes the majority of urinary tract infections, will appear as gram-negative rods (pink rods) under the microscope and Lactobacillus, which is a common vaginal contaminant in women’s urine samples, will appear as gram-positive bacilli (thin purple rods). Some of these bacteria, such as Lactobacillus, are easy for an experienced laboratorian to identify and are nonpathogenic (do not cause an infection) and do not require any further investigation. Others, such as gram-negative rods, represent groups of similar bacteria and will require additional testing to determine exactly which type of bacteria is present.
Based upon the information obtained, the laboratorian gives the doctor an initial report of the quantities and types of bacteria (or yeast) present in the urine. Examples would be: “no growth in 24 hours” (nothing grew on the agar), “less than 10,000 organisms/ml” (a small amount of bacteria was present) or “greater than 100,000 organisms per ml of gram negative rods, ID and susceptibility to follow” (the patient probably has an infection caused by a gram negative bacteria that needs to be further identified).
If there is no or little growth on the agar after 24 to 48 hours of incubation, the urine culture is considered negative for pathogens and the culture is complete. If there is one or more pathogen present, further testing is performed. Biochemical tests are used to identify which bacteria are present and susceptibility testing evaluates the ability of the bacteria to grow in the presence of a specific selection of antimicrobial drugs to help determine which drug treatments are likely to be effective in resolving the infection.
