Definitions

 Sterilisation--the process of destroying all microorganisms (bacteria, viruses, spores, etc.) and their pathogenic products

 Disinfection--the process of destroying most microorganisms (not necessarily spores) on inanimate objects

 Sepsis--the presence of pathogenic microorganisms in blood or tissues

 Asepsis--the absence of pathogenic microorganisms in blood or tissues

 Aseptic technique--steps taken to prevent contact with microorganisms

Sterilisation vs. Disinfection

Pathogenic microorganisms, as well as those that do not normally invade healthy tissue, are capable of causing infection if introduced mechanically into the body. Therefore, specific standardised procedures, based on accepted principles and practices, are necessary for the sterilisation or disinfection of all supplies and equipment used in the operating theatre. Sterilisation renders items safe for contact with tissue without transmission of infection as long as sterility is maintained. Disinfectants are used to kill as many microorganisms in the environment as possible in items and materials that cannot be sterilised. Disinfection can be classified by the ability of the agent to kill microorganisms:

 High-level disinfection kills all bacteria, viruses and fungi. The process may kill spores if contact time is sufficient and other conditions are met.

 Intermediate-level disinfection kills most bacteria, viruses and fungi, but the process does not attack spores.

 Low-level disinfection kills most vegetative bacteria, fungi and the least resistant viruses.

Sterilisation Process

Bacterial spores are the most resistant of all living organisms because of their capacity to withstand external destructive agents. Although the physical or chemical process by which all pathogenic and non-pathogenic microorganisms, including spores, are destroyed is not absolute, supplies and equipment are considered sterile when necessary conditions have been met during a sterilisation process. This is a condition that is difficult to achieve and hard to prove.

Methods of Sterilisation

Reliable sterilisation depends on contact of the sterilising agent with all surfaces of the item to be sterilised. Selection of the agent to achieve sterility depends on the nature of the item to be sterilised. The time required to kill spores is extremely important. Each method of sterilisation has its advantages and disadvantages. (N.B. boiling at 100°C is not considered an effective method of sterilisation and so will not be mentioned during this lecture). Sterilisation processes are either physical or chemical:

 Thermal (physical)

 Steam under pressure--moist heat i.e., autoclave

 Hot air--dry heat

 Microwaves--non-ionising radiation

 Chemical

 Ethylene oxide gas (Anprolene)

 Formaldehyde gas and solution

 Hydrogen peroxide plasma/vapour

 Ozone gas

 Acetic acid solution

 Glutaraldehyde solution

 Peracetic acid solution

 Ionising radiation (physical)

Thermal Sterilisation--Heat

The terms autoclave and steriliser are interchangeable and mean the same thing. 'Autoclave' is a term used mainly in laboratories whilst 'steriliser' is used more commonly for medical and pharmaceutical applications. An effective autoclave or steriliser must contain dry saturated steam. In order to achieve this air must be removed from both the load and the chamber; this can be accomplished in a number of ways. Once all of the air is removed from the load and chamber the temperature within the vessel will rise along with the pressure until the pre-selected temperature is reached.

The temperature within the chamber is reached in a number of ways:

 Steam can be injected into the chamber via an internal steam source such as an integral stainless steel steam generator, which can be built within the autoclave cabinet or can be supplied as a separate unit.

 Some laboratories or hospitals have their own direct steam source, which would similarly be injected, into the chamber.

 In some autoclaves heaters are built into the base of the chamber and the water is heated until it boils and produces steam.

The main advantage of using a steam generator (or the direct steam method) is that cycle times can be considerably faster as the steam is immediately available. In basic autoclaves the water is topped up manually by pouring water into the vessel whilst some autoclaves are connected to a direct water supply and the water level is maintained automatically. Some smaller units are available with an integral water tank, which once filled, can run for up to 20 cycles repeatedly before being replenished. When the desired temperature is achieved for the required time then the steam supply will cease either by shutting off the power to the heaters or by cutting off the steam supply. Thus the temperature and the pressure will gradually drop. In units with a vacuum system fitted the vacuum pump can be used to remove the steam (i.e., where drying is required).

In hot air ovens, death of microbial life by dry heat is a physical oxidation or slow burning process of coagulating the protein in cells. In the absence of moisture, higher temperatures are required than when moisture is present, because microorganisms are destroyed through a very slow process of heat absorption by conduction. Time of exposure varies depending on the load, the individual items and the temperature of the oven.

Chemical Sterilisation

Ethylene oxide gas (EO) is used to sterilise items that are heat or moisture sensitive. EO is a chemical alkylating agent that kills microorganisms, including spores, by interfering with the normal metabolism of protein and reproductive processes, resulting in the death of cells. EO is extremely flammable, toxic and explosive in air, so must be used in a controlled environment.

All packages must be positioned in the chamber to allow free circulation and penetration of gas. Packages should not touch the chamber walls or floor and they should not be stacked tightly. Timing of the cycle is dependent upon the temperature, contents and gas concentration. A cool cycle at 99°F (37°C) may take over 5 hours whereas the same load at 131°F (55°C) may take less than 3 hours. Always follow the manufacturer's instructions.

Adequate aeration for all absorbent materials that will come into contact with skin or tissues is absolutely essential as EO exerts toxic effects on living tissues. Air is admitted to the chamber at the end of the cycle to purge residual gas. Air is admitted then removed as many as six times in 30 minutes, but additional aeration is still required for all wrapped and porous items. Aeration may be performed in room air, or preferably in an aerator provided by the manufacturers. Items must be left to aerate for at least 24 hours after being removed from the chamber.

Monitoring Sterility

For the autoclave, EO gas and hot air oven, there are several indicators to allow us to check the sterility of an item. Biological indicators involve placing bacterial spores within a test pack into the required steriliser and running a cycle. The pack is then left to see whether any growth of bacteria occurs. This test is obviously retrospective and an immediate result is not available. Bowie Dick tape turns from a beige colour to beige with black stripes once a certain temperature is reached, but not for the correct amount of time, so sterility cannot be guaranteed. There is a similar tape for EO gas, which is green, and red stripes appear once contact with EO gas is achieved. There are Anprolene monitors available to check for sterility. Browne's tubes use a traffic light system (i.e., red to green) for use in an autoclave, but different tubes are required for different cycles. The only effective method of autoclave monitoring is the use of TST strips. These strips change from yellow to purple when in contact with steam of the correct temperature and for the correct length of time.