How is it used?
A stool culture is used to detect the presence of disease-causing bacteria (pathogenic) and help diagnose an infection of the digestive tract. It is often used along with other tests such as an O&P test that detects parasites in the stool and/orClostridium difficile and C. difficile toxin tests, to help determine the cause of prolonged diarrhea. In establishing the cause, the test can help guide treatment, determining whether antibiotics or other medication may be necessary to resolve symptoms.
When is it ordered?
Stool cultures may be ordered when someone has signs and symptoms of an infection of the digestive tract. This is especially true when the affected person has eaten food or drunk fluids that the doctor suspect may have been contaminated with pathogenic bacteria, such as undercooked meat or raw eggs, or the same food that has made others ill. Recent travel outside the United States may also suggest possible food contamination. Some signs and symptoms of infection include:
- Diarrhea that lasts more than a few days and may contain blood and/or mucus
- Abdominal pain and cramping
- Nausea, vomiting
Not everyone who has these symptoms will necessarily have testing done or be treated with antibiotics. In people who have healthy immune systems, these infections often resolve with supportive care and without the use of antibiotics. However, if symptoms become severe or are present in an infant, a young child, an elderly person or someone with a weakened immune system, then testing and treatment are usually warranted.
A doctor may order one or more stool cultures when someone has had a previous pathogenic bacterial infection of the gastrointestinal tract and has either been treated for it or it has resolved on its own. This may be done to verify that the pathogenic bacteria are no longer detectable because in some cases people can become carriers of the bacteria. Carriers are not ill themselves, but they can infect other people.
What does the test result mean?
If a stool culture is positive for pathogenic bacteria, then they are the most likely cause of the diarrhea and other symptoms. Results are frequently reported out with the name of the pathogenic bacteria that was detected. Laboratories typically evaluate stool cultures for the most common intestinal pathogens:
- Campylobacter species
- Salmonella species
- Shigella species
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:
- Escherichia coli O157:H7
- Yersinia enterocolitica
- Vibrio species
Certain strains of Clostridium difficile can cause intestinal disease and diarrhea. If these strains are suspected to be the cause, then separate tests that detect the toxin-producing C. difficile will be performed.
Results of stool cultures that are reported as negative usually reflect the fact that the stool culture was checked for the most common pathogens at several intervals and none were found (not isolated). A report may state: “no Campylobacterisolated,” “no Salmonella or Shigella isolated,” etc. If the culture is negative for the major pathogens, then it is likely that the prolonged diarrhea is due to another cause or to a less common pathogen. It is also possible that pathogenic bacteria are present in the gastrointestinal tract, but there were too few bacteria in that particular stool sample to be detected. If a doctor suspects that this is the case and symptoms continue, he may order a stool culture on another sample and/or follow up with other tests.
Most diarrheal disease is caused by a single pathogen, but it is possible to have an infection with more than one.
Is there anything else I should know?
Severe pathogenic bacterial infections of the gastrointestinal tract and those causing complications may be treated with antibiotics, but many uncomplicated cases are best left untreated with antibiotics. Those with healthy immune systems will usually get better on their own within a week or so. People are instructed in how to prevent the spread of the infection and are treated and monitored for symptoms such as dehydration.
Pathogenic bacterial infections are monitored on a community and sometimes national level. Other than foreign travel-related cases, health officials try to determine where an infection came from so that they can address any potential public health concerns. Bacterial isolates causing foodborne illness are sent to the State Public Health Laboratories to be typed by molecular methods. The results of the typing are uploaded into a national database to detect common source outbreaks across the U.S. The typing helps to identify specific foods or food products that are the source of the infection.
Travelers’ diarrhea is typically caused by toxin-producing Escherichia coli, which are the leading cause of bacterial diarrhea infections among travelers to Africa, Asia, and Latin America. These strains of E. coli, however, are different than the strains of Shiga toxin-producing E. coli (O157:H7), which can cause hemolytic uremia syndrome.