In medicine and biotechnology, a sterilizer that uses hot, saturated steam to kill microorganisms is called an autoclave.
This is what an autoclave does.
The hot saturated steam condenses on the cooler sterilized material, releasing its energy. This leads to the coagulation of the protein in the cell and the destruction of the cell[1]. However, not only the moist heat as a heat carrier influences the mortality of an organism. The water content within the cell also plays a decisive role. The lower the water content, the greater the heat resistance. This is why killing bacterial spores is so challenging!
When is something considered sterile?
Sterile means that no viable microorganisms are present anymore (according to EN 556). This includes the killing of microorganisms as well as their dormant stages. In practice, however, this state can never be confirmed with absolute certainty. An acceptable measure for the sterilization of medical devices is that a product can be considered "sterile" if the probability that a viable microorganism is present on or in the product is less than or equal to 1 x 10^-6. To achieve this, certain conditions must be met during sterilization.
The key process parameters are:
- Sterilization time
- Sterilization temperature
- Humidity.
Accurate monitoring of these parameters is crucial for quality assurance.
Advantages of steam sterilization
- Saturated steam with high energy content as an extremely effective sterilizing medium
- Fast and reliable inactivation of microorganisms such as bacteria, fungi, spores, viruses, and parasites
- Uniform distribution of steam throughout the sterilization chamber
- Effective steam penetration even in porous sterilized materials
- Non-toxic, cost-effective, and universally available sterilizing agent
- No residues left on the sterilized goods
Effect of moist heat on microorganisms
In steam sterilization, temperatures are used that exceed the optimal and maximum growth temperature of microorganisms. Empirically, different types of germs can be divided into several resistance levels. Each of these resistance levels is associated with specific disinfection or sterilization procedures.
Examples of heat resistance against moist heat [Wallhäußer, 1988]
Source: Practice of Sterilization, Disinfection, and Preservation. Thieme, Stuttgart 5th edition 1995 (Wallhäuser, K.-H.)
| Resistance Level | Procedure | Organism (Examples) | Temperature (°C) | Time (min.) |
|---|---|---|---|---|
| I | Pasteurization | Pathogenic Streptococci, Listeria, Poliovirus | 61.5 | 30 |
| II | Gentle Heating | Most vegetative bacteria, yeasts, molds, all viruses except Hepatitis B | 80 | 30 |
| III | Boiling | Hepatitis B virus, most fungal spores | 100 | 5 - 30 |
| IV | Pressurized Steam | Bacillus anthracis spores | 105 | 5 |
| V | Steam Sterilization | Bacillus stearothermophilus spores | 121 | 15 |
| VI | Superheated Steam | Clostridium botulinum spores | 135 | 3 |
Table: Resistance of microorganisms against moist heat [Wallhäuser, 1988]
