Sterilization or sterilisation is the process that eliminates, removes, kills, or deactivates all forms of life and other biological agents such as fungi, bacteria, viruses, spore forms, prions, unicellular eukaryotic organisms such as Plasmodium present in a specified region, such as a surface, a volume of fluid, medication, or in a compound such as biological culture media.
Sterilization has a very important role in making and keeping medical devices, implants, operation theaters and other gadgets that are required to be sterile before use.
Sterilization can be achieved through various means, including heat, chemicals, irradiation, high pressure, and filtration.
Sterilization by Heat
Sterilization by Steam
Instruments can be sterilized by steam under pressure using autoclaves. Vegetative bacteria, including tuberculosis, and viruses such as hepatitis B, hepatitis C and human immunodeficiency virus (HIV) and heat-resistant spores, including Clostridium tetani and Clostridium perfringens, are killed. The combination of pressure, temperature and time make them quite susceptible.
The autoclave is sometimes called a converter or steam sterilizer. Autoclaves use steam heated to 121-134 °C under pressure. To achieve sterility, the article is placed in a chamber and heated by injected steam until the article reaches a time and temperature setpoint whose period varies with the amount of burden present on the article being sterilized.
Proper autoclave treatment will inactivate all resistant bacterial spores in addition to fungi, bacteria, and viruses, but is not expected to eliminate all prions.
For prion elimination, various recommendations state 121-132 °C for 60 minutes or 134 °C for at least 18 minutes
Bioindicators like Geobacillus stearothermophilus can also be used to independently confirm autoclave performance.
Moist heat causes the destruction of microorganisms by denaturation of macromolecules, primarily proteins. This method is a faster process than dry heat sterilization
Sterilization by Hot Air
It has the advantage in the ability to treat solid nonaqueous liquids grease/ointments and to process closed airtight containers. Lack of corrosion may be important, particularly with instruments with fine cutting edges such as ophthalmic instruments. It cannot be used for substances such as rubber, plastics and intravenous fluids which are denatured by heat.
Dry heat is a longer process than moist heat sterilization. With longer exposure to lethal temperatures, the number of killed microorganisms increases.
The standard setting for a hot air oven is at least two hours at 160 °C.
Flaming is done to loops and straight-wires in microbiology labs. Leaving the loop in the flame of a Bunsen burner or alcohol lamp until it glows red ensures that any infectious agent is inactivated. This is commonly used for small metal or glass objects, but not for large objects
Heat as a method of sterilization can damage heat-sensitive materials such as biological materials, fiber optics, electronics, and many plastics. Chemical sterilization either by using gases or liquids avoids the problem of heat damage.
But the article to be sterilized should be chemically compatible with the sterilant being used. Moreover, personnel safety measures should be ensured.
Ethylene oxide (EO, EtO) gas treatment is the most common chemical sterilization method because of its wide range of material compatibility. It is also used to process items that are sensitive to processing with other methods, such as radiation, heat or other chemicals.
Ethylene oxide treatment is generally carried out between 30 °C and 60 °C with relative humidity above 30% and a gas concentration between 200 and 800 mg/l.
Nitrogen dioxide is effective sterilant for use against a wide range of microorganisms, including common bacteria, viruses, and spores. It causes degradation of DNA in through nitration of the phosphate backbone, which kills the exposed organism.
The most-resistant organism to sterilization with NO2 gas is the spore of Geobacillus stearothermophilus and is used as a biological indicator in sterilization applications.
Glutaraldehyde and Formaldehyde
Glutaraldehyde and formaldehyde solutions are liquid sterilizing agents but require prolonged immersion.
In a clear liquid, it can take up to 22 hours with glutaraldehyde and even longer with formaldehyde. The presence of solid particles may lengthen the required period. Sterilization of blocks of tissue can take much longer, due to the time required for the fixative to penetrate.
Formaldehyde is also used as a gaseous sterilizing agent. For this, it is prepared on-site by depolymerization of solid paraformaldehyde tablets. Many vaccines, such as the original Salk polio vaccine, are sterilized with formaldehyde.
Hydrogen peroxide, in both liquid and as vaporized hydrogen peroxide, is another chemical sterilizing agent. Hydrogen peroxide is a strong oxidant, which allows it to destroy a wide range of pathogens.
The biggest advantage of hydrogen peroxide as a sterilant is the short cycle time. Whereas the cycle time for ethylene oxide may be 10 to 15 hours, some modern hydrogen peroxide sterilizers have a cycle time as short as 28 minutes.
Drawbacks of hydrogen peroxide include material compatibility, a lower capability for penetration and operator health risks.
Peracetic acid 0.2% is used in sterilizing medical devices such as endoscopes.
Sterilization by Radiation
Sterilization can be achieved using electromagnetic radiation such as electron beams, X-rays, gamma rays, or irradiation by subatomic particles.
Ultraviolet light irradiation is useful for sterilization of surfaces and some transparent objects.
Gamma radiation is very penetrating and is commonly used for sterilization of disposable medical equipment, such as syringes, needles, cannulas and IV sets, and food.
Electron beam processing is also commonly used for sterilization and needs less exposure time is needed. Electron beams are less penetrating than either gamma or x-rays.
X-rays allow irradiation of large packages and pallet loads of medical devices.
Fluids that would be damaged by heat, irradiation or chemical sterilization, such as drug products, can be sterilized by microfiltration using membrane filters. These are commonly made from materials such as mixed cellulose ester or polyethersulfone (PES).
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