What Are Stool Cultures?

Gastrointestinal diseases present with symptoms such as diarrhea, abdominal pain, or blood in the stool. Stool cultures are screening methods for discovering the pathogens that cause them.
What Are Stool Cultures?

Last update: 19 December, 2020

Stool cultures are an effective way to identify digestive system illnesses associated with infections. They’re even a common mechanism for carrying out various scientific studies that conclude the presence of bacteria and parasites in the intestine.

We often become infected with parasites, viruses, and bacteria that generate various symptoms of intestinal malaise. Also, some conditions are expressed by bleeding into feces, such as colitis, gastric cancer, and stomach ulcers. Therefore, a stool culture is the mandatory screening test.

Microbiological cultures

A microbiological culture is a method based on the multiplication of microorganisms in a sterile medium, generally bacteria, in order to facilitate the detection of an illness that causes diseases. In short, it’s to give microorganisms the ideal environment for their development.

The patient’s biological sample plantings come from sputum, saliva rub, skin parts, and also feces. Scientists place them in small containers called petri dishes, with a solid substance inside that contains agar.

Many pathogens have specific growth requirements, so each culture medium is specialized based on the microorganism being sought, as reported by this portal specialized in immunology. Some of these parameters are as follows:

  • Availability of nutrients suitable for bacterial growth
  • Presence or absence of oxygen and other gases
  • Adequate humidity conditions
  • Ambient light
  • Acidity or alkalinity
  • Temperature
  • Sterility of the environment

Depending on the types of microorganisms you want to find, the conditions in the culture media must be different. As soon as there’s growth on the plate, experts isolate samples under a microscope to order to identify the specific species causing the disease.

As various scientific sources point out, there are many different types of stains for the identification of bacteria. Experts place a dye in the stain that the microorganisms take, thus turning them that color. The most common medical use is Gram-type staining.

A petri dish.
Petri dishes are the places where microorganisms grow in laboratories

What is a stool culture?

A stool culture is nothing more than a microbiological culture based on the collection of feces as a sick patient’s sample. It’s used, above all, to study cases of persistent or recurrent diarrhea without a known reason.

As this medical review article states, the collection of fecal samples is necessary in infectious enteric conditions that don’t recede in two or three days. Some of the pathogenic micro-organisms that can be identified by stool cultures are Salmonella, Campylobacter, Helicobacter, Shigella, Yersinia, Clostridium difficile, and also Staphylococcus aureus.

Sample collection

The review article we cite illustrates the steps to take for the collection of the sample from the stool culture:

  1. The patient provides a sample in a wide container with an airtight seal, with a minimum sample volume of two to four grams in pasty feces and five to ten milliliters in liquid feces.
  2. Fecal specimens that have been exposed to the environment for two hours or more, or that are contaminated with urine or toilet paper remains, are not valid.
  3. Scientists take a very small sample from the patient’s feces and plant it in a culture medium suitable for pathogenic microorganisms to grow.
  4. If the first sample doesn’t detect the presence of enteropathogens, it’s necessary to collect two more samples on different days.
  5. Once the pathogenic bacteria have been identified, then the appropriate antibiotic is prescribed.

What to observe stool cultures?

Scientists use these microbiological cultures to identify various illnesses through the identification of bacteria, by planting the fecal sample and subsequent staining. They also examine possible eggs or parasite residues.

According to the Center for Disease Control and Prevention(CDC), parasites such as tapeworms are identified with stool cultures, as their eggs are seen in the feces of the patient. Likewise, if they don’t detect hidden blood straight away in the stools, a biochemist can detect it.

Limitations of stool cultures

Not everything is ideal in the world of microbiological cultures. Results aren’t always possible and are largely dependent on the quality of the sample. As this scientific paper states, there are certain risks when identifying diseases through stool cultures.

First, scientists can’t routinely search for all enteropathogens. Because the culture medium is specific to the microorganisms you want to find, some less common, such as Bacteroides fragilis, Edwarsiella tarda, or Escherichia alberti may not grow.

Also, there are unknown pathogens whose biological requirements we don’t know. Therefore, they may not multiply as they don’t have the right conditions to do so. This is beyond the point-in-time clinical situation, in which the aim is to look for the most common diseases.

Other studies argue that stool cultures are inappropriate for people who have been hospitalized for more than three days. This is because the cause of diarrhea isn’t attributable to pathogens in the intestine. In these cases, the most appropriate method is the PCR detection technique, which identifies specific genes of a microorganism in the fecal sample.

A woman in hospital.
In hospitalizations, stool culture has limitations on use and effectiveness

Stool cultures: what to remember?

These microbiological cultures are useful in identifying pathogenic enteric bacteria, parasites, and also blood in the feces. Still, due to the high cost and delay of the procedure, only patients with specific epidemiological issues can have them.

However, if you need to provide a stool sample in the hospital, there’s no need to worry. Doctors will be trying to accurately identify the cause of gastrointestinal disease, so that they can administer the right drug.

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