Dependence of material on the infection site
To ensure that the bacteriological test results are as accurate as possible, clinical material must be collected directly from the site of the infectious process. Depending on the pathology, the types of biomaterial studied may include pus, various necrotic masses, or biological body fluids.
The choice of a specific substrate is strictly tied to the localization of the inflammatory process:
| Disease (process) | Clinical material |
|---|---|
| Pneumonia | Sputum |
| Cystitis | Urine |
| Meningitis | Cerebrospinal fluid (CSF) |
| Colitis | Feces |
| Abscesses and phlegmon | Pus from the lesion |
Correct selection ensures that the laboratory receives precisely the medium in which the concentration of the target pathogen is maximal.
General principles of collection (asepsis and timing)
The preanalytical stage requires strict adherence to a number of rules. Any mistake at this stage makes subsequent laboratory work meaningless.
- Timing of collection relative to treatment: Clinical material must be obtained strictly before initiating etiotropic therapy (administration of antibiotics or antimycotics). If the drugs have already begun to take effect, bacteriological culture is highly likely to yield a false-negative result.
- Strict asepsis: Only sterile containers are used to collect biomaterial. The main task of the physician and nurse is to completely exclude contamination, i.e., contamination of the sample with environmental microflora, from the patient's own skin, or from the hands of the medical staff. For example, the slightest breach of technique during blood collection leads to skin saprophytes entering the sample. They will grow in the laboratory, leading to an erroneous diagnosis.
- Labeling: Each sample must be carefully labeled. It is mandatory to indicate the patient's full name, date of birth, and the type of material submitted. It is critically important to record the exact time of sample collection, as the assessment of sample suitability for analysis depends on it.
Transportation and the time factor
The delivery of material at stage I of the study must be carried out immediately. Strict time regulations require that the interval from the moment of collection to inoculation onto nutrient media does not exceed 1–2 hours.
There are several reasons for such strict limitations:
- Death of fastidious microorganisms: many pathogens do not survive long outside the host organism.
- Distortion of quantitative analysis: rapidly growing microflora can multiply within a few hours to such an extent that it suppresses the growth of the true pathogen.
- Sample degradation: processes of putrefaction and fermentation begin rapidly in the material.
Sealed containers, tubes, or special transport systems containing supporting nutrient media are used for delivery to maintain bacterial viability.
Special requirements for anaerobes: Non-spore-forming anaerobes are extremely sensitive to atmospheric oxygen and die rapidly upon contact with it. To preserve them, highly specialized methods that completely exclude aeration are used: special transport media or vessels filled with inert gas.
Collection methods in special cases
The technique for obtaining a clinical sample varies depending on the procedure. Among specific methods, the following can be distinguished:
- Duodenal intubation: bile collection is performed fractionally. The material is collected in portions (A, B, and C), each of which is placed in a separate, strictly sterile tube.
- Intraoperative collection: during surgical interventions, material (e.g., pus or exudate) is collected via puncture using a syringe. Maximum asepsis rules are observed, and the material is collected into a single tube.
In both cases, the rule regarding delivery time remains unchanged — no later than 1–2 hours after sample collection.
Briefly: answers to topical questions
What is the exact algorithm for preparing the patient's skin before blood collection for blood culture?
The exact algorithm for skin preparation before blood collection includes double disinfection and strict adherence to aseptic rules.
- Disinfection — the skin is treated twice with 70% alcohol or a 1–2% iodine solution using movements from the center to the periphery.
- Waiting — it is necessary to wait until the disinfectant is completely dry.
- No palpation — after treatment, touching the venipuncture site is strictly prohibited; re-palpation of the vein is not performed.
- Cleansing — after completing the procedure, residual iodine should be removed from the patient's skin.
Which types of bacteria belong to fastidious microflora that die rapidly during transport?
Microorganisms for which sources explicitly indicate rapid death or high sensitivity to environmental/transport conditions include:
- Non-spore-forming anaerobes — die rapidly upon contact with atmospheric oxygen.
- Bordetella — drying of the swab leads to rapid bacterial death; pertussis causative agents are extremely demanding regarding cultivation conditions.
- Meningococcus — extremely labile, unstable in the external environment.
- Mobiluncus — obligate anaerobes, instantly die in air.
The rule of three "T"s: Precise localization, Timely collection before antibiotics, Transportation within 1–2 hours.
Class and exam questions
Why must biological material be collected before starting antibiotic therapy?
Antibacterial and antifungal drugs inhibit the growth of the pathogen. If material is collected during therapy, the culture will show no growth, leading to a false-negative result.
What happens if the specimen is delivered to the laboratory later than 2 hours?
Fastidious microflora may die, while rapidly growing microorganisms, conversely, will actively multiply. This will lead to distorted quantitative analysis results and the activation of putrefaction or fermentation processes.
How to prevent skin saprophytes from contaminating blood cultures?
It is necessary to use only sterile containers and strictly follow aseptic rules during venipuncture to exclude sample contamination with the skin microflora of the patient or staff.
What is the difficulty of collecting specimens for anaerobic infection?
Non-spore-forming anaerobes die upon the slightest contact with oxygen. They must be transported in sealed transport systems or vessels with inert gas.
What else to study on this topic
- Mechanisms of death of fastidious microorganisms outside the body
- Distortion of quantitative analysis by rapidly growing flora
- Duodenal intubation technique and characteristics of portions A, B, C
- Characteristics of transport systems with nutrient media
- The effect of putrefaction and fermentation processes on bacterial viability
Study the full topic in the «Microbiology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "The Preanalytical Stage in Microbiology" and another 4,383 pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 141 video lectures on the subject
Other topics in the «Clinical Microbiology» section
Free video lessons: microbiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.