Sterilization Methods
Sterilization is a type of decontamination where no microbial load remains on an item. There are zero bacteria, fungi, spores or even proteins present to transmit infection. Sterilization is the highest level of decontamination an object can receive. In healthcare settings, it is recommended that all critical and semicritical items are sterilized.
It is important to note that although all decontamination processes aim to reduce microbial load, not all of them result in complete sterilization. Below is a summary of the most common sterilization techniques you might encounter.
Wet Heat (steam)
Steam sterilization using an autoclave is the gold standard method of inactivating all organic pathogens including spores and prion proteins. Some items are heat or pressure-sensitive and as should be sterilized using an alternative appropriate methodology. This technique is commonly used for metal tools and heat resistant plastics.
Dry Heat
Baking objects in an oven is an alternative to wet heat sterilization where an object may be intolerant of moist conditions (for example due to corrosion). The temperature and cycle time is higher than an autoclave (160-190℃ for up to two hours).
Chemical
Chemical sterilants react with proteins and membranes to destroy organic pathogens. Liquid sterilants can also be used to achieve high-level disinfection. These methods eliminate the need for a high temperature so can be a useful alternative to wet and dry heat techniques.
Ethylene oxide chambers use toxic ethylene oxide gas to kill organic pathogens. While useful for moisture and heat-sensitive objects, the byproducts of the process are very environmentally damaging, carcinogenic and explosive. Long degassing times (
Liquid sterilants like peracetic acid and glutaraldehyde require objects to be completely immersed for a verified period of time to achieve sterilization. Immersion of metals in acid for a prolonged period can cause corrosion, while glutaraldehyde will bind blood and proteins irreversibly to the surface of a processed object.
As such careful consideration is needed before using a liquid sterilant, alongside careful pre-cleaning processing.
Irradiation
Both forms of sterilization by irradiation inactivate microorganisms by damaging their DNA either directly or indirectly.
Non-ionizing radiation with ultraviolet (UV) light is a lower energy process than high energy ionizing radiation techniques using gamma rays or X-rays. As such non-ionizing UV radiation can't penetrate objects and can only be used to sterilize surfaces. Any microbes in the direct sightline of the UV bulb will be inactivated due to DNA damage. This is what makes UV light a carcinogen in humans and other animals; skin cell DNA damage by solar UV irradiation is linked to the development of skin cancers.
Irradiation techniques operate at low temperatures, meaning they can be useful for heat-sensitive items.
How do you know if your sterilization run was successful?
There are two types of indicator used to determine if a sterilization method has been completed correctly. These are biological and chemical indicators.
Biological indicators are often small samples of microorganism spores that should be inactivated if the sterilization method is successful. After a run, the spores are plated onto growth medium and checked to see if they produce new cells. A failure to grow is an indication of a good process.
Chemical indicators take the form of color-changing compounds that react if the sterilization conditions are met for the right length of time. These can include ink patches on sterilization pouches that change from pink/yellow to brown/black or adhesive tape that does the same and can be applied to bottles and paper-wrapped parcels.