Indications and patient preparation guidelines
Prescribing a microbiological stool culture has two main goals: identifying the causative agents of acute or chronic inflammatory processes in the intestine (colitis, enteritis) and confirming the diagnosis of dysbiosis.
To ensure reliable results, the patient requires preliminary preparation, which begins 3–4 days before submitting the biomaterial. During this period, it is essential to strictly exclude factors affecting motility and microflora:
- Complete discontinuation of any laxative medications.
- Cessation of the use of rectal suppositories.
- Exclusion of oils from the diet, particularly castor oil and liquid petrolatum.
In laboratory practice, there are strict criteria for sample unsuitability. The sample will be rejected if the faeces were obtained after an enema or after an X-ray examination using barium sulfate.
Algorithm for biomaterial collection and transportation
The collection process requires compliance with strict hygiene standards to prevent contamination of the sample with extraneous flora. Prior to defecation, preliminary urination is mandatory—urine must not get into the stool.
Collection technique into a bedpan:
- The bedpan undergoes thorough processing: disinfection, followed by rinsing with running water, and a final scalding with boiling water.
- Defecation is performed into the prepared bedpan.
- Material is collected using a special spoon only from those areas of the faeces that did not touch the walls of the bedpan.
A sterile disposable container with a screw cap is used for collection. There is also an alternative method: collecting material using a faecal swab or rectal loop, which are then immediately placed into a special transport medium.
Transportation and storage rules:
- Maximum laboratory delivery time is 2 hours.
- Within this timeframe, storing the sample in the cold (in a refrigerator) is permitted.
- It is strictly prohibited: to freeze the sample, to store it for more than 5–6 hours, or to use loosely closed containers.
Laboratory phase and result interpretation
The material received in the laboratory is subjected to quantitative enumeration using the serial dilution method. Inoculation is performed on selective nutrient media designed for the cultivation of specific types of microorganisms, after which the isolated cultures are identified.
Etiological significance criteria in purulent-inflammatory processes:
- Detection in the culture of a microorganism atypical for this localization (intestine).
- A sharp increase in the concentration of one of the representatives of opportunistic microorganisms (OM).
Laboratory criteria for dysbiosis:
- A significant decrease in the number or complete disappearance of representatives of the normal (resident) microflora: bifidobacteria, lactobacilli, typical Escherichia.
- Simultaneous quantitative growth of opportunistic flora, which includes staphylococci, clostridia, as well as fungi of the genus Candida.
Briefly: answers to topical questions
Which microorganisms belong to the obligate (resident) intestinal microflora?
The obligate (resident) intestinal microflora predominantly includes anaerobic bacteria, as well as certain aerobes.
- Bifidobacteria (Bifidobacterium) — main microbiota (up to 10¹² CFU/g).
- Bacteroides (Bacteroides) — main microbiota (up to 10¹¹ CFU/g).
- Lactobacilli (Lactobacillus) — concomitant microbiota (10⁸ CFU/g).
- Escherichia coli (Escherichia coli) — concomitant microbiota (10⁷ CFU/g).
Clostridia, Prevotella, Ruminococcus, Porphyromonas, Enterococci, and Propionibacteria also belong to the resident flora of the large and small intestines.
What selective and differential diagnostic media are used for stool inoculation?
Various elective and differential diagnostic media are used for stool inoculation and the isolation of specific types of microorganisms.
- Blaurock medium — for the isolation of bifidobacteria.
- MRS-2 medium — for the cultivation of lactobacilli.
- Anaerobic blood agar — for Bacteroides.
- Levine or Endo media — for enterobacteria.
- Bile-blood agar — for enterococci.
- Meat-peptone agar with furagin — for Pseudomonas aeruginosa.
- Sabouraud medium — for fungi.
- Yolk-salt agar — for staphylococci.
Which microorganisms belong to the opportunistic flora causing dysbiosis?
The opportunistic microflora, an increase in whose concentration is a criterion for dysbiosis, includes various bacteria and fungi.
- Staphylococci (Staphylococcus) — cause dysbiosis upon a sharp increase in concentration.
- Clostridia (Clostridium) — gram-positive spore-forming anaerobic bacilli.
- Fungi of the genus Candida — yeast-like fungi capable of causing candidiasis when mucosal resistance is reduced.
Dysbiosis is characterized by a simultaneous increase in the concentration of these microorganisms against the background of the disappearance or decrease of the resident microflora.
How is the quantitative enumeration of microorganisms performed using the serial dilution method?
Quantitative enumeration of microorganisms is performed using the serial dilution method.
To remember when you should not submit stool, use the "Two B's" rule: no Barium contrasts and no Bowel cleanses (enemas and laxatives).
Class and exam questions
Is it acceptable to collect stool from the toilet bowl?
No, the material is collected only into a pre-processed bedpan (disinfection, water, boiling water), and areas that did not touch the walls of the bedpan are taken for analysis.
How much time is available to deliver the container to the laboratory?
The container must be delivered within 2 hours. Storage for over 5–6 hours, as well as freezing of the biomaterial, are strictly prohibited.
What changes in flora composition indicate dysbiosis?
The diagnosis is confirmed by a decrease in resident flora levels (Bifidobacterium, Lactobacillus, Escherichia) against the background of a simultaneous increase in opportunistic microorganisms (Clostridium, Staphylococcus, Candida fungi).
What else to study on this topic
- Serial dilution method for quantitative enumeration
- Types of selective culture media for stool inoculation
- Methods for identifying isolated pure cultures
- The role of resident microflora in health and disease
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