Thermal methods
Thermal sterilization is the most common approach. Its effectiveness depends on temperature: while vegetative forms of bacteria die already at 60 °C, spores, due to dense coats and bound water, are inactivated only at 160–170 °C.
- Dry heat. Carried out in air sterilizers (dry heat cabinets, Pasteur ovens). Main mode: 180 °C for 60 minutes (additionally, 160 °C for 150 minutes is used). Only thermostable objects are processed: laboratory glassware, metal instruments, silicone rubber products.
- Pressurized steam (autoclaving). The physical principle is that increasing the pressure in the autoclave shifts the boiling point of water upward. At a pressure of 2 atmospheres, steam temperature reaches 121 °C. The standard mode (121 °C for 15–20 minutes) is guaranteed to kill spores. The method is used for dressings, linens, culture media, solutions, and the disposal of infectious material. Interestingly, hyperthermophilic archaebacteria are capable of multiplying at 100 °C and even withstand an hour of standard autoclaving.
Fractional sterilization (tyndallization)
This method is a lifesaver for thermolabile materials (e.g., culture media containing carbohydrates) that are irreversibly destroyed at temperatures above 100 °C.
The process spans three days. The object is heated daily to 70–80 °C for 30–60 minutes. The first heating destroys vegetative microbial cells. Between heating sessions, the object is placed in an incubator: this prompts surviving spores to germinate and transform into vulnerable vegetative forms. Repeated heatings on subsequent days completely kill this "new wave" of microorganisms.
Chemical and gas processing
- Gas sterilization. Uses toxic gases such as ethylene oxide, OB mixture (ethylene oxide with methyl bromide), or formaldehyde. Gases act as alkylating agents, inactivating proteins and enzymes and damaging nucleic acids. Processing takes place in sealed chambers at 20–60 °C in the presence of steam. Due to the high penetrating power of gases, the method is ideal for optics, polymers, electronics, and protein media.
- Endoscope processing. Complex equipment made of thermolabile materials is subjected to immersion. They are filled with solutions of chemical substances (sterilants), most commonly formaldehyde or glutaraldehyde. After sterilization, the device must be rinsed free of the solution, dried, and stored under strict aseptic conditions for no more than 3 days.
Physical methods
- Radiation sterilization. Widely used in industry for processing huge batches of single-use medical devices (syringes, blood transfusion sets). The active agent is a beam of accelerated electrons or $\gamma$-radiation from radioactive isotopes. Radiation fatally damages microbial nucleic acids.
- Filtration sterilization (mechanical method). Used to free thermolabile liquids from microorganisms: blood serum, drug solutions, culture media. The liquid is passed through ceramic, asbestos, or glass filters. Membrane nitrocellulose filters are considered the "gold standard" in this group.
Promising technologies also include sterilization using ozone and ionized plasma.
Efficiency control
Four groups of methods are used to check the quality of equipment operation:
- Microbiological. Inoculation of a portion of the sterilized object onto culture media (aerobes, anaerobes, fungi). The method is highly accurate, but due to its duration, it is not used in routine practice.
- Chemical. Use of indicators—substances that change color or physical state at the required temperature (ampoules with benzoic acid or urea, indicator strips). They are placed both on the surface and deep inside objects.
- Biological (biotests). Use of spores of highly resistant bacteria (Bacillus stearothermophilus, Bacillus licheniformis). Spores are hidden inside items; if they are dead, the mode worked.
- Technical. Regular inspection of equipment operability by specialized services.
Briefly: answers to topical questions
What standard autoclaving modes are used in medicine?
Gentle and harsh autoclaving modes are used in medicine, as well as specific modes for prion inactivation.
| Mode | Temperature | Pressure | Time | Objects of sterilization |
|---|---|---|---|---|
| Gentle | 120 °C | 1.1 atm | 45 min | Wipes, gowns, rubber drains, gloves, suture material |
| Harsh | 130–132,9 °C | 2 atm | 20 min | Surgical linen, dressings, surgical instruments |
Special conditions are required for prion inactivation: 121 °C for 4 hours or 134 °C for 30 minutes.
Which chemicals are used for the chemical immersion sterilization of endoscopes?
Solutions of aldehydes and other liquid chemical reagents are used for the chemical immersion sterilization of endoscopes.
- Formaldehyde is the primary aldehyde for special equipment.
- Glutaraldehyde is used as a sterilant, as well as a base for fixatives and preservatives.
- Chlorhexidine is used as an alcohol solution.
- Formic-peracetic acid mixture (Pervomur) is used for pouring over optical instruments until fully covered.
Which substances are used as chemical melting indicators during thermal sterilization?
Powdered substances with a specific melting point, placed between layers of material, are used as chemical melting indicators.
To monitor steam sterilization (autoclaving), the following are used:
- Benzoic acid melts at 120 °C.
- Resorcinol melts at 119 °C.
- Antipyrine melts at 110 °C.
To monitor dry-heat sterilization (in dry-heat cabinets), the following are used:
- Ascorbic acid melts at 187–192 °C.
- Succinic acid melts at 180–184 °C.
- Thiourea melts at 180 °C.
- Pilocarpine hydrochloride melts at 200 °C.
What active agent is used in plasma sterilization?
Plasma sterilization uses ionized plasma.
Tyndallization works like a military ambush: the first heating destroys the "active infantry" (vegetative forms). During the incubation pause, spores germinate, emerging from their secure shelters, and the next heating finishes them off.
Class and exam questions
Why is regular boiling not enough for sterilization?
At a temperature of 100 °C, bacterial spore forms can remain viable for up to 5 hours. Guaranteed destruction requires a temperature of 121 °C (pressurized steam) or 160–170 °C (dry heat).
What is tyndallization and why is it needed?
This is a method of fractional sterilization at 70–80 °C, repeated for 3 consecutive days. It allows thermolabile items, which are destroyed at high temperatures, to be completely cleared of microbes and their spores.
How is optical equipment (e.g., endoscopes) sterilized?
Complex heat-sensitive optics are sterilized chemically: either by immersion in aldehyde solutions (formaldehyde, glutaraldehyde) or by gas sterilization in specialized chambers.
What else to study on this topic
- Mechanisms of extreme prion resistance to high temperatures
- Biology of hyperthermophilic archaebacteria surviving in the autoclave
- Operating principles and design of nitrocellulose membrane filters
- Mechanism of action of alkylating agents (gases) on microbial nucleic acids
- Specific application of Bacillus stearothermophilus spores in biological monitoring
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