- Importance of stool culture
- Process
- Procedure for taking the sample
- Laboratory procedure
- Selective means to obtain the results
- Normal test values
- Repeat stool culture
- References
The stool is a method of microscopic analysis of stool. This serves to identify the pathogenic batteries that cause gastrointestinal diseases. It is also known as a coproparasitological test.
In the normal intestinal flora, there are microorganisms that do not cause diseases and help to digest food. This is the case of anaerobes (gram-positive rods and cocci, bacteroides). In contrast, gram-negative enteric organisms and Enterococcus faecalis are capable of causing disease.
Other pathogens capable of causing gastrointestinal diseases are viruses, toxins, invasive enteric gram-negative rods, slow lactose fermenters, shigella and salmonella, and campylobacteria.
The culture is done by placing a gel on the stool. Then, the pathogenic bacteria that grow in it are observed for identification, as well as the sensitivity and / or resistance reaction to antibiotics.
The indiscriminate use of antibiotics without a prescription causes resistance to pathogens in the patient. Therefore, a prior examination and determination of the cause of the digestive disorder is important.
The indication of antidiarrheal drugs in patients with a diagnosis of infectious liquid stools is not recommended. These drugs keep the pathogen inside the body and can cause adverse events in the patient.
Importance of stool culture
When the patient presents any symptoms of gastrointestinal problems, the doctor may request an examination to make an accurate diagnosis.
Some of the symptoms to request the test are:
- Liquid stools (diarrhea) for more than 3 or 4 days.
- Mucus or bloody stools
- Constant emetic episodes (vomiting)
- Feverish state
- Stomach gas in greater quantity than normal
- Stomach pains and cramps
The determination of the contaminating bacteria or organism will allow the doctor to indicate the appropriate treatment.
Sometimes it can be due to a viral condition. In that case, the indication of antibiotics is not warranted. These would have no effect on treatment, and may cause pathogenic resistance over time.
Among the enteric bacterial pathogens that cause common gastrointestinal diseases are:
- Shigella
- Salmonella
- Escherichia coli
- Yersenia
- Campylobacter
Process
Stool culture can be considered a routine test, in which infections caused by enteric bacterial pathogens are detected.
Procedure for taking the sample
- The patient should empty his bladder before performing the test to avoid contaminating it with urine.
- Place a plastic container or bag inside the toilet to collect the sample.
- Put on gloves to collect the sample.
- Transfer the feces from the plastic container to the collection container that will be taken to the laboratory with a plastic pallet that will later be discarded.
- Prevent collected feces from touching the toilet to avoid contamination with other infectious agents.
- Avoid touching stool with soap or toilet paper.
- Close the sample collector for the laboratory and properly identify.
- Wash hands
- Take to the laboratory as soon as possible to avoid degradation of the sample.
Laboratory procedure
- The laboratory technician places the stool samples on sterile plates with a substance that causes bacteria to grow.
- They are kept at a temperature suitable for the accelerated growth of the target bacteria.
- Bacterial colonies are not expected to form. Then it is considered a negative result, and consequently normal.
- If bacterial colonies do form, the test is positive. The technician will need to look under the microscope and perform some chemical tests to identify the microorganism.
- The result is expected in a period of 24 to 48 hours.
Selective means to obtain the results
The use of the media for the determination of the most common enteric bacterial pathogens causing gastrointestinal diseases are as follows:
- A selective and differential medium, such as MacConkey or Eosin metylene blueagar, is used to detect Salmonella and Shigella, since they inhibit gram-positive organisms.
- Some species of enteric bacteria such as Salmonella and Shigella do not ferment lactose. However, many other enteric gram-negative rods are lactose fermenters.
- If the latter are identified, an iron triple sugar agar is indicated.
- To differentiate Salmonella from Proteus on triple iron sugar agar, the enzyme urease is analyzed. This is not produced by Salmonella but by Proteus.
- Campylobacter jejuni grows in culture media such as Skirrow Agar, at 42 ° C.
- Anaerobic cultures are generally not indicated, since anaerobes rarely become pathogens in the gut. However, a large amount of anaerobes are observed in the stool.
Normal test values
“Normal” bacteria are found in the flora of the gastrointestinal tract that play an important role in the digestion of food. They also prevent the growth of pathogenic organisms.
The expected Normal Value is a negative response to the appearance of bacteria, viruses or fungi.
An Abnormal Value is one where colonies of bacteria or fungi are observed. This warrants its identification for the beginning of the suitable treatment.
If, despite negative stool culture results, diarrhea persists, the clinician should consider clinical reevaluation. This can establish non-bacterial causes such as parasites, inflammatory bowel disease, adverse effects to medication, among others.
A stool culture lab report generally carries the following aspects:
- Appearance of the feces: the type of consistency presented in the sample is observed. It can be liquid, pasty, or formed.
- PH type: acidic, alkaline or neutral.
- Stool color: brown, whitish, gray or black.
- Microscopic analysis: plasma if mucus, leukocytes, erythrocytes, or parasites are observed.
- Culture analysis: it is determined if the growth of any colony of bacteria is observed. If positive, the type is indicated.
Repeat stool culture
In some cases, it is necessary to repeat the stool culture. The reasons may be the following:
- The patient did not notify the treating physician of the recent X-ray with contrast material, and / or the recent use of broad-spectrum antibiotics.
- Stool samples taken to the laboratory with mixed urine.
- Inadequate collection process.
- Sample degraded by time.
Stool culture is a laboratory test that has an important role in the investigation of patients with suspected infectious diarrhea.
Determining the pathogenic microorganisms present provides the treating physician with guidance for timely and effective treatment.
References
- American Association for Clinical Chemistry. Stool Culture. Taken from labtestsonline.org.
- Healthline Media. Stool Culture. Taken from healthline.com
- NorthShore University HealthSystem. Stool analysis. Taken from northshore.org
- University of Rochester Medical Center Rochester. Stool Culture. Taken from: urmc.rochester.edu
- S. National Library of Medicine. Stool culture. Taken from: medlineplus.gov.