Process that eliminates all biological agents on an object or in a volume
Sterilization () refers to any process that removes, kills, or deactivates all forms of life (particularly microorganisms such as fungi, bacteria, spores, and unicellular eukaryotic organisms) and other biological agents (such as prions or viruses) present in fluid or on a specific surface or object. Sterilization can be achieved through various means, including heat, chemicals, irradiation, high pressure, and filtration. Sterilization is distinct from disinfection, sanitization, and pasteurization, in that those methods reduce rather than eliminate all forms of life and biological agents present. After sterilization, fluid or an object is referred to as being sterile or aseptic.
Applications.
Foods.
One of the first steps toward modernized sterilization was made by Nicolas Appert, who discovered that application of heat over a suitable period of time slowed the decay of foods and various liquids, preserving them for safe consumption for a longer time than was typical. Canning of foods is an extension of the same principle and has helped to reduce food borne illness ("food poisoning"). Other methods of sterilizing foods include ultra-high temperature processing (which uses a shorter duration of heating), food irradiation, and high pressure (pascalization).
In the context of food, sterility typically refers to "commercial sterility", "the absence of microorganisms capable of growing in the food at normal non-refrigerated conditions at which the food is likely to be held during distribution and storage" according to the "Codex Allimentarius".
Medicine and surgery.
In general, surgical instruments and medications that enter an already aseptic part of the body (such as the bloodstream, or penetrating the skin) must be sterile. Examples of such instruments include scalpels, hypodermic needles, and artificial pacemakers. This is also essential in the manufacture of parenteral pharmaceuticals.
Preparation of injectable medications and intravenous solutions for fluid replacement therapy requires not only sterility but also well-designed containers to prevent entry of adventitious agents after initial product sterilization.
Most medical and surgical devices used in healthcare facilities are made of materials that are able to undergo steam sterilization. However, since 1950, there has been an increase in medical devices and instruments made of materials (e.g., plastics) that require low-temperature sterilization. Ethylene oxide gas has been used since the 1950s for heat- and moisture-sensitive medical devices. Within the past 15 years, a number of new, low-temperature sterilization systems (e.g., vaporized hydrogen peroxide, peracetic acid immersion, ozone) have been developed and are being used to sterilize medical devices.
Spacecraft.
There are strict international rules to protect the contamination of Solar System bodies from biological material from Earth. Standards vary depending on both the type of mission and its destination; the more likely a planet is considered to be habitable, the stricter the requirements are.
Many components of instruments used on spacecraft cannot withstand very high temperatures, so techniques not requiring excessive temperatures are used as tolerated, including heating to at least , chemical sterilization, oxidization, ultraviolet, and irradiation.
Quantification.
The aim of sterilization is the reduction of initially present microorganisms or other potential pathogens. The degree of sterilization is commonly expressed by multiples of the decimal reduction time, or D-value, denoting the time needed to reduce the initial number formula_1 to one tenth (formula_2) of its original value. Then the number of microorganisms formula_3 after sterilization time formula_4 is given by:
formula_5.
The D-value is a function of sterilization conditions and varies with the type of microorganism, temperature, water activity, pH, etc.. For steam sterilization (see below), typically the temperature, in degrees Celsius, is given as an index.
Theoretically, the likelihood of the survival of an individual microorganism is never zero. To compensate for this, the overkill method is often used. Using the overkill method, sterilization is performed by sterilizing for longer than is required to kill the bioburden present on or in the item being sterilized. This provides a sterility assurance level (SAL) equal to the probability of a non-sterile unit.
For high-risk applications, such as medical devices and injections, a sterility assurance level of at least 10−6 is required by the United States of America Food and Drug Administration (FDA).
Heat.
Steam.
Steam sterilization, also known as moist heat sterilization, uses heated saturated steam under pressure to inactivate or kill microorganisms via denaturation of macromolecules, primarily proteins. This method is a faster process than dry heat sterilization. Steam sterilization is performed using an autoclave, sometimes called a converter or steam sterilizer. The object or liquid is placed in the autoclave chamber, which is then sealed and heated using pressurized steam to a temperature set point for a defined period of time. Steam sterilization cycles can be categorized as either pre-vacuum or gravity displacement. Gravity displacement cycles rely on the lower density of the injected steam to force cooler, denser air out of the chamber drain. Steam Sterilization | Disinfection &amp; Sterilization Guidelines | Guidelines Library | Infection Control | CDC In comparison, pre-vacuum cycles create a vacuum in the chamber to remove cool dry air prior to injecting saturated steam, resulting in faster heating and shorter cycle times. Typical steam sterilization cycles are between 3 and 30 minutes at at , but adjustments may be made depending on the bioburden of the article being sterilized, its resistance (D-value) to steam sterilization, the article's heat tolerance, and the required sterility assurance level. Following the completion of a cycle, liquids in a pressurized autoclave must be cooled slowly to avoid boiling over when the pressure is released. This may be achieved by gradually depressurizing the sterilization chamber and allowing liquids to evaporate under a negative pressure, while cooling the contents.
Proper autoclave treatment will inactivate all resistant bacterial spores in addition to fungi, bacteria, and viruses, but is not expected to eliminate all prions, which vary in their heat resistance. For prion elimination, various recommendations state for 60 minutes or for at least 18 minutes. The 263K scrapie prion is inactivated relatively quickly by such sterilization procedures; however, other strains of scrapie and strains of Creutzfeldt-Jakob disease (CKD) and bovine spongiform encephalopathy (BSE) are more resistant. Using mice as test animals, one experiment showed that heating BSE positive brain tissue at for 18 minutes resulted in only a 2.5 log decrease in prion infectivity.
Most autoclaves have meters and charts that record or display information, particularly temperature and pressure as a function of time. The information is checked to ensure that the conditions required for sterilization have been met. Indicator tape is often placed on the packages of products prior to autoclaving, and some packaging incorporates indicators. The indicator changes color when exposed to steam, providing a visual confirmation.
Biological indicators can also be used to independently confirm autoclave performance. Simple biological indicator devices are commercially available, based on microbial spores. Most contain spores of the heat-resistant microbe "Geobacillus stearothermophilus" (formerly "Bacillus stearothermophilus"), which is extremely resistant to steam sterilization. Biological indicators may take the form of glass vials of spores and liquid media, or as spores on strips of paper inside glassine envelopes. These indicators are placed in locations where it is difficult for steam to reach to verify that steam is penetrating that area.
For autoclaving, cleaning is critical. Extraneous biological matter or grime may shield organisms from steam penetration. Proper cleaning can be achieved through physical scrubbing, sonication, ultrasound, or pulsed air.
Pressure cooking and canning is analogous to autoclaving, and when performed correctly renders food sterile.
To sterilize waste materials that are chiefly composed of liquid, a purpose-built effluent decontamination system can be utilized. These devices can function using a variety of sterilants, although using heat via steam is most common.
Dry.
Dry heat was the first method of sterilization and is a longer process than moist heat sterilization. The destruction of microorganisms through the use of dry heat is a gradual phenomenon. With longer exposure to lethal temperatures, the number of killed microorganisms increases. Forced ventilation of hot air can be used to increase the rate at which heat is transferred to an organism and reduce the temperature and amount of time needed to achieve sterility. At higher temperatures, shorter exposure times are required to kill organisms. This can reduce heat-induced damage to food products.
The standard setting for a hot air oven is at least two hours at . A rapid method heats air to for 6 minutes for unwrapped objects and 12 minutes for wrapped objects. Dry heat has the advantage that it can be used on powders and other heat-stable items that are adversely affected by steam (e.g., it does not cause rusting of steel objects).
Flaming.
Flaming is done to inoculation loops and straight-wires in microbiology labs for streaking. Leaving the loop in the flame of a Bunsen burner or alcohol burner until it glows red ensures that any infectious agent is inactivated or killed. This is commonly used for small metal or glass objects, but not for large objects (see Incineration below). However, during the initial heating, infectious material may be sprayed from the wire surface before it is killed, contaminating nearby surfaces and objects. Therefore, special heaters have been developed that surround the inoculating loop with a heated cage, ensuring that such sprayed material does not further contaminate the area. Another problem is that gas flames may leave carbon or other residues on the object if the object is not heated enough. A variation on flaming is to dip the object in a 70% or more concentrated solution of ethanol, then briefly leave the object in the flame of a Bunsen burner. The ethanol will ignite and burn off rapidly, leaving less residue than a gas flame
Incineration.
Incineration is a waste treatment process that involves the combustion of organic substances contained in waste materials. This method also burns any organism to ash. It is used to sterilize medical and other biohazardous waste before it is discarded with non-hazardous waste. Bacteria incinerators are mini furnaces that incinerate and kill off any microorganisms that may be on an inoculating loop or wire.
Tyndallization.
Named after John Tyndall, tyndallization is an obsolete and lengthy process designed to reduce the level of activity of sporulating microbes that are left by a simple boiling water method. The process involves boiling for a period of time (typically 20 minutes) at atmospheric pressure, cooling, incubating for a day, and then repeating the process a total of three to four times. The incubation allow heat-resistant spores surviving the previous boiling period to germinate and form the heat-sensitive vegetative (growing) stage, which can be killed by the next boiling step. This is effective because many spores are stimulated to grow by the heat shock. The procedure only works for media that can support bacterial growth, and will not sterilize non-nutritive substrates like water. Tyndallization is also ineffective against prions.
Glass bead sterilizers.
Glass bead sterilizers work by heating glass beads to . Instruments are then quickly doused in these glass beads, which heat the object while physically scraping contaminants off their surface. Glass bead sterilizers were once a common sterilization method employed in dental offices as well as biological laboratories, but are not approved by the U.S. Food and Drug Administration (FDA) and Centers for Disease Control and Prevention (CDC) to be used as a sterilizers since 1997. They are still popular in European and Israeli dental practices, although there are no current evidence-based guidelines for using this sterilizer.
Chemical sterilization.
Chemicals are also used for sterilization. Heating provides a reliable way to rid objects of all transmissible agents, but it is not always appropriate if it will damage heat-sensitive materials such as biological materials, fiber optics, electronics, and many plastics. In these situations, chemicals either in a gaseous or liquid form, can be used as sterilants. While the use of gas and liquid chemical sterilants avoids the problem of heat damage, users must ensure that the article to be sterilized is chemically compatible with the sterilant being used and that the sterilant is able to reach all surfaces that must be sterilized (typically cannot penetrate packaging). In addition, the use of chemical sterilants poses new challenges for workplace safety, as the properties that make chemicals effective sterilants usually make them harmful to humans. The procedure for removing sterilant residue from the sterilized materials varies depending on the chemical and process that is used.
Ethylene oxide.
Ethylene oxide (EO, EtO) gas treatment is one of the common methods used to sterilize, pasteurize, or disinfect items because of its wide range of material compatibility. It is also used to process items that are sensitive to processing