The Test Sample
What is being tested?
The stool culture is a test that detects and identifies bacteria that cause infections of the lower digestive tract. The test distinguishes between the types of bacteria that cause disease (pathogenic) and the types that are normally found in the digestive tract (normal flora). The test helps to determine if pathogenic bacteria are the cause of gastrointestinal symptoms (gastroenteritis).
The bacteria found in stool are representative of the bacteria that are present in the gastrointestinal tract. Certain bacteria and fungi called normal flora inhabit everyone's gastrointestinal tract. They play an important role in the digestion of food, and their presence keeps a check on the growth of disease-causing bacteria. Sometimes, the balance of the normal flora may be affected by the administration of broad-spectrum antibiotics; the drugs inhibit the growth of normal flora and allow bacteria resistant to the antibiotic to survive and overgrow the digestive tract. The normal flora may also be affected by anti-cancer drugs and when a person has a weakened immune system, leading to bacterial overgrowth and symptoms such as diarrhea and abdominal pain.
Pathogenic bacteria can enter and infect the digestive tract when someone eats food or drinks water that is contaminated. Examples of contaminated sources include raw or undercooked eggs, poultry or beef, unpasteurized milk, and untreated water from lakes, streams, and (occasionally) from community water supplies.
People who travel outside the U.S., especially to developing nations, may face a greater risk of being exposed to disease-causing bacteria. Some of these bacteria may be true pathogens while others are strains of gastrointestinal bacteria that are normal flora for the local inhabitants but cause gastrointestinal distress to the tourist. Visitors may become infected by eating or drinking anything that has been contaminated with the bacteria, even things as simple as tap water, ice cubes in a drink, a fresh salad, or food from a vendor's stall.
The most common symptoms of a pathogenic bacterial infection are prolonged diarrhea, bloody diarrhea, mucus in the stool, abdominal pain and cramping, and nausea. If diarrhea lasts more than a few days, it may lead to complications such as dehydration and electrolyte imbalance - dangerous conditions, especially in children and the elderly. Dehydration can cause symptoms such as dry skin, fatigue, and light-headedness.
Severely affected people may require hospitalization to replace lost fluids and electrolytes. Hemolytic uremic syndrome is a serious complication characterized by the destruction of red blood cells and kidney failure that may occasionally arise from an infection with a toxin-producing strain of the bacteria Escherichia coli. The condition is most frequently seen in children, the elderly, and those with weakened immune systems.
Some of the most common pathogenic bacteria that cause infections in the U.S. and their most frequently encountered sources include:
- Salmonella - found in raw eggs (even intact disinfected eggs), raw poultry uncooked vegetables, and in reptiles; pets, such as lizards and turtles, may carry salmonella in their intestines without being ill themselves. Some humans may become carriers of salmonella. Salmonella may be transmitted person-to-person.
- Shigella - found in food and water contaminated with stool, and from infected-person to person when careful sanitation is not observed; for instance, it can be a challenge to prevent the spread of Shigella within a family and in a daycare or nursing home setting since even a very few organisms may cause disease.
- Campylobacter - found in raw or undercooked poultry; it is the most common cause of bacterial diarrhea in the U.S. It may become especially serious if it spreads to the bloodstream, and it occasionally causes long-term complications such as arthritis and Guillain-Barré syndrome.
- Escherichia coli 0157:H7 and other toxin-producing E. coli (most strains of E. coli are considered normal flora and do not cause disease) - found in raw or undercooked hamburger/beef, spinach, or unpasteurized cider; causes bloody diarrhea and may lead to hemolytic uremic syndrome.
- Clostridium difficile - may be present as part of the normal flora, but use of broad-spectrum antibiotics can result in an overgrowth of these bacteria. Toxin-producing strains can cause diarrhea and other serious complications.
A wide variety of other bacteria may sometimes cause diarrhea. These may include: Aeromonas species, Staphylococcus aureus, Yersinia enterocolitica, Vibrio cholerae and other Vibrio species.
How is the sample collected for testing?
A fresh stool sample is collected in a sterile container. The stool sample should not be contaminated with urine or water. Once it has been collected, the stool should be taken to the laboratory within about an hour after collection or should be transferred into a vial containing a preservative and taken to the lab as soon as possible. For infants, a stool sample is usually collected with a swab of the rectum.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.