Sterilization (microbiology)
Sterilization (
Applications
Foods
One of the first steps toward modernized sterilization was made by
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.[5]
Medicine and surgery
In general, surgical instruments and medications that enter an already
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.[6]
Most medical and surgical devices used in healthcare facilities are made of materials that are able to go under steam sterilization.[7] 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.[8]
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.[9]
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 120 °C (248 °F), chemical sterilization, oxidization, ultraviolet, and irradiation.[10]
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 to one tenth () of its original value.[11] Then the number of microorganisms after sterilization time is given by:
- .
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.[citation needed]
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.[citation needed]
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 Food and Drug Administration (FDA).[12]
Heat
Steam
Steam sterilization, also known as moist heat sterilization, uses heated
Proper autoclave treatment will inactivate all resistant bacterial
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.[16]
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.[17]
Pressure cooking and canning is analogous to autoclaving, and when performed correctly renders food sterile.[18][failed verification]
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.[citation needed]
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.[19]
The standard setting for a hot air oven is at least two hours at 160 °C (320 °F). A rapid method heats air to 190 °C (374 °F) for 6 minutes for unwrapped objects and 12 minutes for wrapped objects.[20][21] 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
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.[22]
Tyndallization
Named after
Glass bead sterilizers
Glass bead sterilizers work by heating glass beads to 250 °C (482 °F). 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
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,
Ethylene oxide
Ethylene oxide treatment is generally carried out between 30 and 60 °C (86 and 140 °F) with
The traditional process consists of a preconditioning phase (in a separate room or cell), a processing phase (more commonly in a vacuum vessel and sometimes in a pressure rated vessel), and an aeration phase (in a separate room or cell) to remove EO residues and lower by-products such as
The most common EO processing method is the gas chamber method. To benefit from
Ethylene oxide is still widely used by medical device manufacturers.
It is important to adhere to patient and healthcare personnel government specified limits of EO residues in and/or on processed products, operator exposure after processing, during storage and handling of EO gas cylinders, and environmental emissions produced when using EO.
The U.S.
Nitrogen dioxide
The most-resistant organism (MRO) to sterilization with NO2 gas is the spore of
Ozone
Ozone offers many advantages as a sterilant gas; ozone is a very efficient sterilant because of its strong oxidizing properties (
Glutaraldehyde and formaldehyde
Hydrogen peroxide
Drawbacks of hydrogen peroxide include material compatibility, a lower capability for penetration and operator health risks. Products containing cellulose, such as paper, cannot be sterilized using VHP and products containing
Vaporized hydrogen peroxide (VHP) is used to sterilize large enclosed and sealed areas, such as entire rooms and aircraft interiors.
Although toxic, VHP breaks down in a short time to water and oxygen.
Peracetic acid
Peracetic acid (0.2%) is a recognized sterilant by the FDA[49] for use in sterilizing medical devices such as endoscopes. Peracetic acid which is also known as peroxyacetic acid is a chemical compound often used in disinfectants such as sanitizers. It is most commonly produced by the reaction of acetic acid and hydrogen peroxide with each other by using an acid catalyst. Peracetic acid is never sold in unstabilized solutions which is why it is considered to be environmentally friendly.[50] Peracetic acid is a colorless liquid and the molecular formula of peracetic acid is C2H4O3 or CH3COOOH.[51] More recently, peracetic acid is being used throughout the world as more people are using fumigation to decontaminate surfaces to reduce the risk of COVID-19 and other diseases.[52]
Potential for chemical sterilization of prions
Prions are highly resistant to chemical sterilization.[53] Treatment with aldehydes, such as formaldehyde, have actually been shown to increase prion resistance. Hydrogen peroxide (3%) for one hour was shown to be ineffective, providing less than 3 logs (10−3) reduction in contamination. Iodine, formaldehyde, glutaraldehyde, and peracetic acid also fail this test (one hour treatment).[54] Only chlorine, phenolic compounds, guanidinium thiocyanate, and sodium hydroxide reduce prion levels by more than 4 logs; chlorine (too corrosive to use on certain objects) and sodium hydroxide are the most consistent. Many studies have shown the effectiveness of sodium hydroxide.[55]
Radiation sterilization
Sterilization can be achieved using
Non-ionizing radiation sterilization
Ultraviolet light irradiation (UV, from a
Ionizing radiation sterilization
The safety of irradiation facilities is regulated by the International Atomic Energy Agency of the United Nations and monitored by the different national Nuclear Regulatory Commissions (NRC). The radiation exposure accidents that have occurred in the past are documented by the agency and thoroughly analyzed to determine the cause and improvement potential. Such improvements are then mandated to retrofit existing facilities and future design.
Use of a radioisotope requires shielding for the safety of the operators while in use and in storage. With most designs, the radioisotope is lowered into a water-filled source storage pool, which absorbs radiation and allows maintenance personnel to enter the radiation shield. One variant keeps the radioisotope under water at all times and lowers the product to be irradiated in the water in hermetically sealed bells; no further shielding is required for such designs. Other uncommonly used designs use dry storage, providing movable shields that reduce radiation levels in areas of the irradiation chamber. An incident in Decatur, Georgia, US, where water-soluble caesium-137 leaked into the source storage pool, requiring NRC intervention[58] has led to use of this radioisotope being almost entirely discontinued in favour of the more costly, non-water-soluble cobalt-60. Cobalt-60 gamma photons have about twice the energy, and hence greater penetrating range, of caesium-137-produced radiation.
Sterilization by irradiation with gamma rays may however affect material properties.[61][62]
Irradiation is used by the United States Postal Service to sterilize mail in the Washington, D.C. area. Some foods (e.g. spices and ground meats) are sterilized by irradiation.[63]
Subatomic particles may be more or less penetrating and may be generated by a radioisotope or a device, depending upon the type of particle.
Sterile filtration
Fluids that would be damaged by heat, irradiation or chemical sterilization, such as drug solution, can be sterilized by
Membrane filters used in production processes are commonly made from materials such as mixed
Preservation of sterility
Instruments that have undergone sterilization can be maintained in such condition by containment in sealed packaging until use.
Aseptic technique is the act of maintaining sterility during procedures.
See also
- Antibacterial soap
- Asepsis
- Aseptic processing
- Contamination control
- Electron irradiation
- Food packaging
- Food preservation
- Food safety
- Spaulding classification
- Sterilant gas monitoring
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