Direct (bacteriological) method
This approach is considered the most accurate, as it directly confirms the absence of viable microflora. It requires time for laboratory cultivation and is carried out in two variants.
Material swab method The procedure is performed directly in the operating room. Medical personnel open the sterilization drum (drum container). Sterile gauze pieces pre-moistened with isotonic sodium chloride solution (Natrii chloridum) are used for sampling. The processed linen surface is thoroughly swabbed with this gauze, after which it is placed in a test tube and transported to the bacteriological laboratory for inoculation.
Biological indicators (bacteriological tests) Special test tubes are used for this method. Inside them is a known non-pathogenic culture of spore-forming bacteria. The characteristic feature of this test culture is that it is guaranteed to die only at a strictly defined temperature.
- Placement: Test tubes with the test culture are placed deep inside the drum before processing begins.
- Evaluation: Upon completion of the cycle, the test tubes are retrieved and sent to biologists. If microbial growth is absent, the material is considered completely sterile.
- Frequency: Such in-depth testing is organized once every 10 days.
Indirect (chemical) method
Indirect monitoring is carried out continuously—during absolutely every sterilization procedure. It allows for a prompt assessment of equipment performance.
The essence of using chemical indicators lies in using special powdered substances with a fixed melting point.
Application technique: Ampoules or glass test tubes are filled with 0.5 grams of the selected chemical substance. The top of the container is closed with a gauze plug. Usually, 1–2 such tubes are placed deep between the layers of the material being sterilized. The main criterion for success is the melting of the powder into a solid mass. This means that the temperature inside the drum has reached or exceeded the melting point of the control substance.
Depending on the type of equipment, different reagents are used:
| Equipment type | Chemical substance | Melting point |
|---|---|---|
| Autoclaves | Antipyrine | 110°C |
| Resorcinol | 119°C | |
| Benzoic acid | 120°C | |
| Dry-heat sterilizers | Thiourea | 180°C |
| Succinic acid | 180-184°C | |
| Ascorbic acid | 187-192°C | |
| Pilocarpine hydrochloride | 200°C |
Thermometry and general room monitoring
In addition to fusible indicators, indirect methods include thermometry. It is considered a more objective way to evaluate the temperature regime. One or two maximum thermometers are placed inside the drum along with the materials. After removal, they show the highest temperature reached inside the container. However, this method has a significant drawback: it does not record exposure time (the duration this temperature was maintained). For this reason, thermometry does not eliminate the need for regular direct bacteriological control.
Comprehensive sterility control in the department To ensure safety, not only instruments but also the environment of the surgical unit is evaluated. For this purpose, bacteriological studies of the air are carried out, and swabs are taken from the ceiling, walls, and medical devices.
- General routine sampling from surfaces and materials is organized once a month.
- Selective cleanliness control (sampling from the hands of medical personnel) is carried out weekly.
Briefly: answers to topical questions
Which specific microbial strains are used in biological indicators for sterility control?
Heat-resistant spore-forming bacilli are used in biological indicators for sterility control. The following microbial strains are applied:
- Bacillus stearothermophilus (Bacillus stearothermophilus) — spores are placed inside the items being sterilized.
- Bacillus licheniformis (Bacillus licheniformis) — spore death indicates the effectiveness of the chosen regimen.
What classes of chemical indicators exist for sterility control?
The materials do not describe the classes of chemical indicators, but rather their types based on color change or physical state. The following types of chemical tests exist:
- Indicator strips — change color when certain parameters are reached.
- Powders — substances with a specific melting point that turn into a solid mass.
- Liquids in ampoules — contain chemical compounds (e.g., urea).
Benzoic acid, resorcinol, and antipyrine are used for autoclaves. Ascorbic and succinic acids, pilocarpine hydrochloride, and thiourea are used for dry-heat sterilizers.
How to memorize autoclave indicators (low temperatures 110–120°C)? The acronym «BRA» in Russian: Benzoic acid, Resorcinol, Antipyrine. For dry-heat sterilizers (high temperatures 180–200°C) — «AYAPT»: Ascorbic, Succinic, Pilocarpine, Thiourea.
Class and exam questions
Which sterility control method is the most accurate?
The direct bacteriological method is recognized as the most reliable. It provides a definitive answer regarding the presence or absence of viable microorganisms on the processed material.
How often must biological indicators with spores be used?
Test tubes with a known non-pathogenic spore-bearing culture are placed inside sterilization drums once every 10 days.
What is the main drawback of using maximum thermometers?
A maximum thermometer shows only the peak temperature reached, but cannot record the exposure time at these values.
How often is the sterility of the surgical team's hands checked?
Bacteriological swabs from the hands of medical personnel are taken selectively every week.
What else to study on this topic
- Technique for proper bacteriological sampling from surgical linen
- Types of non-pathogenic spore-forming cultures for biological tests
- Principles of distributing chemical indicators between material layers
- Evaluation of exposure time under various autoclaving regimens
- Algorithm of actions upon detecting microbial growth in control cultures
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