Moist heat sterilization
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.
Simply Boiling 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 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.
Action on micro-organisms
Moist heat causes destruction of micro-organisms by denaturation 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.