Sterilization
Sterilization means the freeing of an article from all organisms, including viruses, bacteria, and their spores, fungi and their spores, both pathogenic and non-pathogenic. It is an absolute germ-free state. Sterilization is required for culture media, suspending fluids, reagents, containers, and equipment used in the laboratory.
Methods of Sterilization
The following main methods are used for sterilization:
1. Heat: Heat is applied in its two forms i.e., dry heat and moist heat. It is a very reliable and widely-applied method. Temperatures above 100°C, under controlled conditions, kill spores as well.
2. Ionising Radiation Beta (ȕ electrons) and *amma (Ȗ photons) irradiation are used in the industry for disposable, single-use items such as needles, syringes, latex catheters, and surgical gloves.
3. Filtration: Used to remove bacteria from fluids, which are spoiled by heating, e.g. blood, semen, and antibiotic solutions.
4. Chemical Disinfectants: These can be
a. Gases: Ethylene oxide is mainly used in the industry for the sterilization of heat-sensitive material, which cannot withstand heating (such as plastics).
b. Liquids: Certain liquids, such as glutaraldehyde, can be used when no other sterilization method is available. These are not very effective or reliable.
Sterilization by Dry Heat
Dry heat is suitable for glassware, instruments & paper-wrapped articles that are not spoiled by very high temperatures, and for water-impermeable oils, waxes, and powders. Dry heat cannot be used for water-containing culture media. The methods for the application of dry heat include:
1. Red Heat: The articles to be sterilized are put directly in the flame, until red-hot. It has its application in the sterilization of inoculating wires and loops, tips of needles and forceps. These should be held vertically in the flame until they are red-hot along their whole length.
2. Flaming: This means the direct exposure of articles to gas or spirit flames. This method, however, does not ensure complete sterilization.
3. Hot-air Oven: This mode of heat is applied to substances which can withstand high temperatures in the range of 160-180°C and cannot be reliably penetrated by moist heat. It is used for glassware such as tubes, flasks, measuring cylinders, all-glass syringes, and glass pipettes, powders, oils and greases in sealed containers. A Hot-air Oven is also used for the sterilization of:
a. Glassware
b. Forceps, scalpels, scissors. etc.
c. Throat swabs
d. Syringes
e. Dry materials in sealed containers
f. Powders, fats, oils, and greases which are impermeable to moisture.
The following precautions should be observed when using a hot-air oven:
(i). The oven must not be overloaded. Space must be left for the circulation of air through the articles.
(ii). It must first be loaded and then heated up to the sterilization temperature in the course of 1-2 hours.
(iii). A holding period of one hour at 160°C is required for sterilization. (this means for one hour after attaining 160°C).
4. Infra-Red Radiation: Infra-red rays are generated by an electric element and these rays are allowed to fall on the objects to be sterilized. The object is heated and sterilized.
Sterilization by Moist Heat. The methods of applying moist heat include:
1. Pasteurization: This method is used for sterilizing milk. The temperature required is either 63-66°C for 30 minutes. By this method, the eating utensils, clothing, and bedsheets of patients can also be sterilized.
2. Boiling: Simple boiling is used for sterilizing articles like syringes. Moist heat contained at 100°C continuously for 90 minutes is used to sterilize culture media. Intermittent exposure at 100°C for 20-30 minutes for three consecutive days is called Tyndallization. This is used for materials which are destroyed or denatured by prolonged heat, such as media containing sugars. It allows for the killing of germinating spores.
3. Steaming (below 100°C): Steaming below 100°C is used for delicate materials.
4. Steaming (above 100°C): Moist heat at this temperature is achieved using heat under pressure. The equipment used for this purpose is called an autoclave. A household pressure cooker is a good example of a simple autoclave.
5. Steaming above 100°C (under pressure): This is the most effective method of sterilization and requires an autoclave.
