In our last series of posts we reviewed the options for manual cleaning and disinfection , as well as the options for touchless disinfection technologies to complement manual disinfection. In the next series of posts we will dive more deeply into what makes up some of the disinfection agents from sterilizers to disinfectants, to sanitizers and antimicrobials as well as how they work and what they can kill.
To kick off this series we are going to start with sterilizers, or sterilants. We know that sterilizers are “A substance, or mixture of substances, that destroys or eliminates all forms of microbial life in the inanimate environment, including all forms of vegetative bacteria, bacterial spores, fungi, fungal spores, and viruses. These products are commonly used in hospitals, laboratories, pharmaceutical clean rooms, and similar environments where sterilization is necessary” (1). If we look at this in more depth the key term here is that the substance or mix of substances should be able to “eliminates all forms of microbial life in the inanimate environment, including all forms of vegetative bacteria, bacterial spores, fungi, fungal spores, and viruses.”
Sterilents are recommended for use by the CDC HICPAC committee in Critical areas-those areas that are used to enter sterile tissue or the vascular system. A wide range of instruements and medical devices are therefore required to be sterilized. We are going to look in more depths at those sterilizers below.
Sterilization can be broadly divided into a physical method or chemical method.
Physical methods-Heat and Time.
These are normally sterilization conducted under pressure (autoclaves) or dry heat ovens. Typically medical and surgical devices are sterilized this way as they are heat stable. Heat is a well recognized agent for microbial deactivation. Heat destroys microorganisms by coagulation and denaturation of enzymes and structural proteins.
Dry heat sterilizers are preferred for metal instruments as the heat alone does not dull instrument edges, and as there is no water or fluid corrosion or rust will not occur. Heat and time are the primary microbiocidal mode of action. There are two types
- “Toaster oven”-this style is a slower sterilize requiring around an hour to sterilize the devices which are sterilized at 320oF
- COX rapid heat sterilizer-this proprietary system of forced air and baffles uses dry heat to provide sterilization in as little as 6-12 minutes;
- Cycle 1: 6 minutes for unwrapped instruments
- Cycle 2: 8 minutes for hand pieces & medical drills
- Cycle 3: 12 minutes for packaged instruments.
Autoclaves or pressure sterilization are preferred when you want to sterilize a wide range of materials ranging from contaminated materials to medical devices. These units use a combination of temperature, pressure and time to sterilize the contents. The pressure in the autoclaves are a method to generate high temperatures, as heat and time are the primary microbiocidal mode of action (121oC (250oF) or 132oC (270oF). Autoclaves traditionally are either;
- gravity displacement autoclaves (steam is admitted at the top or sides of the chamber, and as lighter than air forces air out of the chamber)
- less efficient then high speed-a wrapped medical load takes 30 minutes at 121oC (250oF)
- high speed prevacuum autoclaves (a vacuum pump or ejector forces the air from the sterilization chamber and then steam is added)
- more efficient- a wrapped medical load takes 4 minutes at 132oC (270oF) in a prevacuum sterilizer
Chemical methods-Oxidization, alkylation,
Chemical methods allow sterilization at low temperatures, especially for equipment that is temperature or moisture sensitive. A range of chemical sterilizers are available as outlined below;
100% Ethylene Oxide (ETO), and ETO mixtures
A widely used chemical sterilient in use since the 1950s, the key parameters are gas concentration (450-1200mg/l), temperature, humidity and time. Microbicidal activity of ETO is considered to be the result of alkylation of proteins, DNA, and RNA. Alkylation, or the replacement of a hydrogen atom with an alkyl group, within cells prevents normal cellular metabolism and replication. Ethylene oxide sterilization consists of 5 stages and can take up to 14 hours for a full cycle of all 5 stages;
- Preconditioning and humidification-the chamber is prepared for ideal sterilization conditions-air is removed, and steam injected to increase humidity as this increases ETO activity
- Gas introduction-ETO is injected with steam to keep up humidity
- Exposure-The ETO and steam is maintained at the right concentration as ETO can be absorbed by plastics
- Evacuation-ETO is removed from the chamber
- Air washes-HEPA filtered air is circualted over the load at 30-50oC
There are two main types of ETO used;
- 100% ETO
- mixed gases (ETO mixtures)
- ETO-carbon dioxide (CO2) mixture consists of 8.5% ETO and 91.5% CO2.
- ETO-HCFC mixtures- 8.6% ETO and 91.4% HCFC, and the other mixture is composed of 10% ETO and 90% HCFC
Hydrogen Peroxide Gas Plasma
A patented process where gas plasma is generated within an enclosed chamber under a deep vacuum. Hydrogen peroxide is injected from a cassette and vaporized within the sterilization chamber at 6mg/l diffusing throughout the chamber and exposing all the surfaces of the load to the hydrogen peroxide . An electic field created by radio frequencies creates a gas plasma generating hydroxyl and hydroperoxyl free radicals within the plasma. The gas is removed and vented via high pressure filtered air. This process produces water vapor and oxygen which can then be safely handled and reduces the need for repeated ventilation as in ETO sterilization. Processing time is typically around 73 minutes, but with new changes to the unit has dropped to 52 minutes.
Microbiocidal activity is via hydrogen peroxide chemical oxidation of cellular components, and generation of free radicals to further the oxidation of microbes.
Peracetic acid is mostly used for medical device reprocessing-namely endoscopes. Due to recently noted concerns around reprocessed endoscopes and an outbreak of CRE , the FDA has published updated guidelines for reprocessing medical devices . Peraceteic acid is favored due to its efficacy to not be affected by organic soil loads, and is used in a automated machine with a final peraceteic acid concentration of 0.2% at 50oC. The solution is exposed to the channels of the endoscope for 12 minutes, and then rinsed four times with water.
Microbiocidal action is thought to occur via the oxidative action of the peraceteic acid whcih denatures proteins, disrupts cell wall membrane permeability, and oxidises bonds in metabolites.
CDC HICPAC has prepared a nice overview of the advantages and disadvantages of each sterilization method discussed above (available here).
As always please feel free to add your thoughts and comments below.
References cited in this article
- EPA Pesticide Registration Manual-Chapter 4-available here
Picture references and credits