What is stool culture?
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 a person’s gastrointestinal symptoms (gastroenteritis).
The bacteria found in stool are representative of the bacteria that are present in the digestive system (gastrointestinal tract). Certain bacteria and fungi called normal flora to 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 the bacteria Clostridium difficile that is resistant to the antibiotics to survive and overgrow the digestive tract, leading to 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. (For more on this, see the article Food and Waterborne Illness.)
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, which can be dangerous conditions, especially for 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. The 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.
If a person’s illness is uncomplicated and goes away within a few days, a healthcare practitioner may not order testing. However, if symptoms are severe, if there is bloody diarrhea or mucus present in the stool, or if it is continuing unabated, then a stool culture may be ordered. This is especially true if the person has been outside the U.S. and/or has eaten or drunk anything that has also made someone close to them ill.
To aid diagnosis, a stool culture may be done in conjunction with or following a GI pathogens panel that simultaneously tests for multiple disease-causing bacteria, viruses, and parasites. Other tests that may be done include an ova and parasite exam or antigen tests to identify specific microbes.
How is the sample collected for testing?
A fresh stool sample is collected in a clean 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 two hours 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.
How is it used?
A stool culture is used to detect the presence of disease-causing (pathogenic) bacteria and help diagnose an infection of the digestive system (gastrointestinal, GI tract). Since there are many causes of GI infections, a stool culture may be used in conjunction with other tests, such as a GI pathogens panel or an ova and parasite exam (O&P) to help establish a diagnosis.
A GI pathogens panel is performed on a stool sample to simultaneously test for the viruses, bacteria, and parasites that are common causes of GI infections. These panels are relatively new and laboratories and healthcare practitioners are still determining how to utilize them to provide the optimum patient results while being cost-effective. A stool culture and an O&P may be done if a microbe is not identified by the panel and/or there is a suspicion of bacteria or parasites other than those included in the GI panel.
Many GI infections in the U.S. resolve with no specific treatment, with only supportive care, and may not require testing. In otherwise healthy individuals, the infections are considered common illnesses that are not serious and sometimes thought of as “food poisoning” or
“stomach flu.” However, there are cases where it is useful to perform testing that identifies the cause of the GI infection – to guide its treatment, eliminate its source, and limit its spread.
Laboratories typically use stool cultures to detect and identify the most common intestinal disease-causing bacteria:
- Campylobacter species
- Salmonella species
- Shigella species
Some bacteria cause illness by producing toxins. These bacteria may be cultured, but many of the tests used to detect them are dedicated to the direct detection of their toxins, either by PCR or antigen test. Examples include:
- Escherichia coli O157:H7
- Clostridium difficile
Depending on the affected person’s medical and/or travel history, tests for other pathogens or potential pathogens may be included or be separately available. Some of these include:
- Yersinia enterocolitica
- Vibrio species