Nome formal
Culture, stool
Este artigo foi revisto pela última vez em
Este artigo foi modificado pela última vez em
13 de Janeiro de 2018.
At a Glance
Why Get Tested?
To determine whether you have pathogenic bacteria in your gastrointestinal tract
When To Get Tested?
When you have diarrhea that lasts more than a few days and/or have blood or mucus in your loose stools
Sample Required?
A fresh stool sample or one that has been placed in a transport medium (a collection vial containing a preservative)
Test Preparation Needed?
None
What is being tested?
The stool culture is a test that allows the detection and identification of pathogenic bacteria in the stool. In the laboratory, the test is initiated by applying a small amount of a fresh fecal sample to a variety of nutrient media. These thin layers of gelatin-like material in sterile, covered plastic dishes allow the growth of potential pathogens and discourage the growth of normal bacteria. Once inoculated with stool, the media are incubated and checked daily for bacterial growth. Bacteria that are present in the stool grow as colonies that look like dots on the surface of the gel. The physical characteristics of the colonies - their shape, color, and some of their chemical properties are unique to each type of bacteria and allow them to be differentiated.

The bacteria in the stool are representative of the bacteria that are present in the gastrointestinal tract. Bacteria and fungi called normal flora inhabit everyone’s gastrointestinal tract. They play an important role in the digestion of food, and they form a protective barrier against the growth of pathogenic bacteria. The balance of the normal flora may be upset by the administration of broad-spectrum antibiotics, which inhibit the growth of normal flora and allow bacteria resistant to the antibiotic to survive.

Pathogenic bacteria enter the body when someone eats food or drinks water that has been contaminated. This may include raw or undercooked eggs, poultry or beef, unpasteurized milk, and contaminated 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 a 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 nausea. If diarrhea lasts more than a few days, it may lead to dehydration and electrolyte imbalance - dangerous conditions, especially in children and the elderly. Dehydration can cause symptoms such as dry skin, fatigue, light-headedness, and fever. Severely affected patients may require hospitalization to replace lost fluids and electrolytes. Hemolytic uremic syndrome (the destruction of red blood cells and kidney failure) is a serious complication that may occasionally arise from an infection with a toxin-producing bacteria, Escherichia coli. It is most frequently seen in children and the elderly.

The most common pathogenic bacteria seen in the stool and their most frequently encountered sources include:

  • Salmonella, often found in raw eggs (even intact disinfected eggs), raw poultry 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, from fecally contaminated food and water, 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 very few organisms may cause disease.
  • Campylobacter, from 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). Found in raw or undercooked hamburger/beef, spinach, or unpasteurized cider. Causes bloody diarrhea and may lead to hemolytic uremic syndrome.
Other bacteria that may cause diarrhea include: Staphylococcus aureus, Clostridium difficile, Yersinia enterocolitica, and 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. The vial should be labeled with the patient’s name and the date and time of the stool collection.

Special measures will need to be taken with diaper-wearing infants, both to prevent urine contamination of the sample and to prevent the samples from touching the inside surface of disposable diapers. The diapers often contain a bacteriostatic agent that will inhibit the growth of the bacteria in the sample and interfere with the results of the stool culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.
Accordion Title
Common Questions
  • How is it used?
    A stool culture is used, often along with other tests such as an O&P test that detects parasites in the stool and/or a Clostridium difficile toxin test, to help determine the cause of your prolonged diarrhea.
  • When is it ordered?
    Stool cultures may be ordered when you have had diarrhea for several days and when you have blood and/or mucus in your loose stools. This is especially true when you have eaten food or drunk fluids that you or your doctor suspect may have been contaminated with a 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. When your doctor suspects that you may have a parasitic infection, he may also order an O&P test. When your diarrhea begins during or after antibiotic treatment, then your doctor may order a Clostridium difficile toxin test along with a stool culture.

    If you have had a previous pathogenic bacterial infection of your gastrointestinal tract and have been treated for it or gotten better on your own, your doctor may order one or more stool cultures to verify that the pathogenic bacteria are no longer detectable. This can be important 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?
    Results are frequently reported out with the name of the pathogenic bacteria and whether it was “isolated” (found in your stool sample) or “not isolated” (that bacteria was not found). Negative results usually reflect the fact that the stool culture was checked for pathogens at several intervals and none were found (not isolated). A report may say something like: “no Campylobacter isolated”, “no Salmonella or Shigella isolated”, etc. If the culture is negative for the major pathogens, then it is likely that your diarrhea is due to another cause. It is also possible that the pathogenic bacteria are present in the gastrointestinal tract but were not found in this particular stool sample. If your doctor suspects this is the case and your symptoms continue, he may order another stool culture.

    If your stool culture is positive for pathogenic bacteria, then that is the most likely cause of your prolonged diarrhea. The stool culture report may state “Salmonella enteritidis isolated” (which means that you have an infection caused by this particular pathogenic bacterium). 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 untreated with antibiotics. Patients with competent immune systems will usually get better on their own within a week or so. Patients 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 level. Other than travel-related cases, health officials want to try to determine where your infection came from so that they can address any potential public health concerns. For instance if your salmonella or shigella infection is due to eating food from a particular restaurant, the health department will want to investigate whether or not other people have also become ill from their food. They will visit the restaurant to determine the source of the infection and take steps to ensure that the spread of the infection is stopped.

  • What can be done to prevent a bacterial infection?
    The best things to do are to not drink water or eat food that may be contaminated and to be careful with sanitation measures such as hand washing. Food that might be contaminated, such as raw meats and eggs, should be cooked thoroughly. Cooked foods and foods that are served raw should not touch any surfaces that may have been contaminated. When you are traveling to developing nations, it is best to only drink bottled water, carbonated drinks, and hot cooked foods. Avoid fresh fruits and vegetables, limiting yourself to those that you can peel yourself. Food from food vendors is generally not considered safe. If someone in your household has an infection that is causing diarrhea, careful hand washing by all family members is recommended, and it may be best to have the person infected not prepare food or drink for others until the infection is over.
  • Why must the stool sample be fresh?
    Bacteria in the stool will continue to multiply. If the stool is not fresh, or in a preservative, the proportion of the different kinds of bacteria in the stool can change – no longer representing the proportions present in the gastrointestinal tract. Overgrowth of normal bacteria can sometimes prevent the detection of the pathogenic bacteria as can exposing the stool sample to temperature extremes.
  • Why shouldn’t I take an over the counter anti-diarrhea medicine when I have infectious diarrhea?
    You should only take this on the advice of your doctor. Diarrhea is one of the methods your body uses to help rid itself of the infection. If you slow down or prevent this from happening by taking anti-diarrhea medication, you can prolong the amount of time that you are ill and sometimes make your infection worse.
  • Once I’ve had a pathogenic bacterial infection, can I be re-infected?
    Generally yes. You may develop a short-term immunity against the particular strain of pathogenic bacteria that caused your infection, but there are many other types and strains of pathogenic bacteria that can make you ill if you are exposed to them.
  • What else can cause diarrhea?
    Diarrhea can be due to a viral infection, parasitic infection, food intolerance, due to certain medications (directly causing diarrhea or indirectly by decreasing normal flora), to a bowel disease or bowel dysfunction. Diarrhea may also be caused or exacerbated by psychological stresses.
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