Stool Culture

The stool culture is a test that allows the detection and identification of pathogenic (harmful) bacteria in the stool. In the laboratory, a small amount of a fresh faecal sample is applied to a variety of nutrient media (thin layers of gelatin like material in sterile covered plastic dishes). These media are selective, each encourages the growth of some bacteria and discourages the growth of others. Once inoculated, 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, colour, 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 and it is a far from sterile environment. Bacteria and fungi called “normal flora” inhabit the gastrointestinal tract. They play an important role in the digestion of food and they form a protective barrier against the growth of other pathogenic bacteria. Normal flora are usually in balance but sometimes one, such as Clostridium difficile, will become pathogenic by overgrowing. This upsets the balance of the normal flora and may lead to the production of toxins that can irritate and damage the intestinal tract. Bacterial overgrowth is usually a side effect of the administration of broad-spectrum antibiotics (which depresses the growth of the other normal flora) but it may also be seen in patients who are immunocompromised.

Other pathogenic bacteria are usually brought into the body when someone eats food or drinks water that has been contaminated. This may include raw or undercooked eggs, poultry or beef, unpasteurised milk, and contaminated water from lakes, streams, and (occasionally) from community water supplies. Those that travel outside the U.K., especially to developing nations, may face a greater risk of being exposed to pathogenic bacteria. Some of these bacteria may be “true pathogens” while others are strains of gastrointestinal bacteria that are normal flora for the locals, but cause gastrointestinal distress to the tourist. Visitors may become infected by eating or drinking anything that has been contaminated with the bacteria (things as simple as tap water, ice cubes in a drink, a fresh salad etc).

The most common symptoms of a pathogenic bacterial infection are prolonged, bloody diarrhoea, mucous in stool, abdominal pain, and nausea. This is usually a self-limiting illness and doesn’t require antibiotics. If diarrhoea lasts more than a few days, it may lead to dehydration and electrolyte imbalance - dangerous conditions in children and the elderly. Dehydration can cause symptoms such as: thirst, dry skin, fatigue, light-headedness, and fever. Severely affected patients may require hospitalization to replace lost fluids and replace lost electrolytes. A serious complication that may occasionally arise is haemolytic uraemic syndrome. It is most frequently seen in children infected with Eschericia coli 0157:H7 and may lead to the destruction of red blood cells and to kidney failure.

The most common pathogenic bacteria 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.

• Shigella, from faecally 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 day care or nursing home setting.
• Campylobacter, from raw or undercooked poultry. It is the most common cause of bacterial diarrhoea in the U.K. It may become especially serious if it spreads to the bloodstream and occasionally causes long-term complications such as arthritis, and Guillain-Barré syndrome (an autoimmune condition affecting the nervous system), however this is rare.
• Escherichia coli 0157:H7 (other strains of E. coli are normal flora). Found in raw or undercooked hamburger / beef or unpasteurised cider. Causes bloody diarrhoea and may lead to haemolytic uraemic syndrome.

Others include: Staphylococcus aureus, Clostridium difficile and strains of Yersinia and Vibrio.

How is the sample collected for testing?

A fresh stool sample is collected in a sterile universal container. The stool sample should not be contaminated with urine or water. Once it has been collected the stool should be sent to the laboratory as soon as possible. The container should be labelled with the patient’s forename, surname, date of birth (as a minimum) and the date and time of the stool collection.

Special measures will need to be taken with infants wearing a nappy, both to prevent urine contamination of the sample and to prevent the samples from touching the inside surface of disposable nappies. The nappies 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.