Stool Culture

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.