Moist heat sterilization
Encyclopedia
Heating an article is one of the earliest forms of sterilization
practiced. Moist heat, as the name indicates, utilizes hot air that is heavily laden with water vapour and where this moisture plays the most important role in the process of sterilization.
a sample for 30 minutes or more will kill virtually all vegetative
cells present, but will not kill spores, which can germinate shortly thereafter and resume growth. Therefore, boiling is an insufficient method to achieve sterilisation
.
A more effective method is Tyndallisation, which uses three successive steam treatments to achieve sterilisation over the course of three days. This works by killing vegetative cells, allowing germination of surviving spores, and killing the resulting vegetative cells before they have time to form further spores. Any surviving spores from the first treatment, or incidentally formed spores during the first incubation period, are killed in a third steaming cycle.
A more commonly used method when extended heat is not a concern is to use an autoclave
or pressure cooker
. When sterilising in this way, samples are placed into a steam chamber on a shelf or raised floor, and the chamber is closed and heated so that steam forces air out of the vents or exhausts. Pressure is then applied so that the interior temperature reaches 121°C, and this temperature is maintained for between 15 and 30 minutes. This elevated temperature and pressure is sufficient to sterilise samples of any commonly encountered microbes or spores. The chamber is then allowed to cool slowly or by passive heat dissipation; it is rare for forced cooling to be applied, or for pressure to be vented deliberately.
Pressure sterilisation is the prevailing method used for medical sterilisation of heat-resistant tools, and for sterilisation of materials for microbiology and other other fields calling for aseptic technique
. To facilitate efficient sterilisation by steam and pressure, there are several methods of verification and indication used; these include colour-changing indicator tapes and biological indicators.
For any method of moist heat sterilisation, it is common to use biological indicators as a means of validation and confirmation. When using biological indicators, samples containing spores of heat-resistant microbes such as Geobacillus stearothermophilis
are sterlilised alongside a standard load, and are then incubated in sterile media (often contained within the sample in a glass ampoule to be broken after sterilisation). A colour change in the media (indicating acid production by bacteria; requires the medium to be formulated for this purpose), or the appearance of turbidity
(cloudiness indicating light scattering by bacterial cells) indicates that sterilisation was not achieved and the sterilisation cycle may need revision or improvement.
of macromolecules, primarily proteins. Destruction of cells by lysis may also play a role. While "sterility" implies the destruction of free-living organisms which may grow within a sample, sterilisation does not necessarily entail destruction of infectious matter. Prions are an example of an infectious agent that can survive sterilisation by moist heat, depending on conditions.
Sterilization (microbiology)
Sterilization is a term referring to any process that eliminates or kills all forms of microbial life, including transmissible agents present on a surface, contained in a fluid, in medication, or in a compound such as biological culture media...
practiced. Moist heat, as the name indicates, utilizes hot air that is heavily laden with water vapour and where this moisture plays the most important role in the process of sterilization.
Methods used
Simply BoilingBoiling
Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. While below the boiling point a liquid...
a sample for 30 minutes or more will kill virtually all vegetative
Vegetative
Vegetative describes vegetation.Vegetative may also refer to:*Vegetative reproduction, a type of asexual reproduction for plants*Persistent vegetative state, a condition of people with severe brain damage...
cells present, but will not kill spores, which can germinate shortly thereafter and resume growth. Therefore, boiling is an insufficient method to achieve sterilisation
Sterilization (microbiology)
Sterilization is a term referring to any process that eliminates or kills all forms of microbial life, including transmissible agents present on a surface, contained in a fluid, in medication, or in a compound such as biological culture media...
.
A more effective method is Tyndallisation, which uses three successive steam treatments to achieve sterilisation over the course of three days. This works by killing vegetative cells, allowing germination of surviving spores, and killing the resulting vegetative cells before they have time to form further spores. Any surviving spores from the first treatment, or incidentally formed spores during the first incubation period, are killed in a third steaming cycle.
A more commonly used method when extended heat is not a concern is to use an autoclave
Autoclave
An autoclave is an instrument used to sterilize equipment and supplies by subjecting them to high pressure saturated steam at 121 °C for around 15–20 minutes depending on the size of the load and the contents. It was invented by Charles Chamberland in 1879, although a precursor known as the...
or pressure cooker
Pressure cooking
Pressure cooking is a method of cooking in a sealed vessel that does not permit air or liquids to escape below a preset pressure. Because the boiling point of water increases as the pressure increases, the pressure built up inside the cooker allows the liquid in the pot to rise to a higher...
. When sterilising in this way, samples are placed into a steam chamber on a shelf or raised floor, and the chamber is closed and heated so that steam forces air out of the vents or exhausts. Pressure is then applied so that the interior temperature reaches 121°C, and this temperature is maintained for between 15 and 30 minutes. This elevated temperature and pressure is sufficient to sterilise samples of any commonly encountered microbes or spores. The chamber is then allowed to cool slowly or by passive heat dissipation; it is rare for forced cooling to be applied, or for pressure to be vented deliberately.
Pressure sterilisation is the prevailing method used for medical sterilisation of heat-resistant tools, and for sterilisation of materials for microbiology and other other fields calling for aseptic technique
Aseptic technique
Aseptic technique refers to a procedure that is performed under sterile conditions. This includes medical and laboratory techniques, such as with microbiological cultures. It includes techniques like flame sterilization...
. To facilitate efficient sterilisation by steam and pressure, there are several methods of verification and indication used; these include colour-changing indicator tapes and biological indicators.
For any method of moist heat sterilisation, it is common to use biological indicators as a means of validation and confirmation. When using biological indicators, samples containing spores of heat-resistant microbes such as Geobacillus stearothermophilis
Bacillus stearothermophilus
Bacillus stearothermophilus is a rod-shaped, Gram-positive bacterium and a member of the division Firmicutes. The bacteria is a thermophile and is widely distributed in soil, hot springs, ocean sediment, and is a cause of spoilage in food products. It will grow within a temperature range of 30-75...
are sterlilised alongside a standard load, and are then incubated in sterile media (often contained within the sample in a glass ampoule to be broken after sterilisation). A colour change in the media (indicating acid production by bacteria; requires the medium to be formulated for this purpose), or the appearance of turbidity
Turbidity
Turbidity is the cloudiness or haziness of a fluid caused by individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality....
(cloudiness indicating light scattering by bacterial cells) indicates that sterilisation was not achieved and the sterilisation cycle may need revision or improvement.
Action on micro-organisms
Moist heat causes destruction of micro-organisms by denaturationDenaturation
Denaturation may refer to:*Denaturation , a structural change in macromolecules caused by extreme conditions*Denaturation , transforming fissile materials so that they cannot be used in nuclear weapons...
of macromolecules, primarily proteins. Destruction of cells by lysis may also play a role. While "sterility" implies the destruction of free-living organisms which may grow within a sample, sterilisation does not necessarily entail destruction of infectious matter. Prions are an example of an infectious agent that can survive sterilisation by moist heat, depending on conditions.