Sterilization: is defined as the process where all the living microorganisms, including bacterial spores are killed. Disinfection: is the process of elimination of most pathogenic microorganisms (excluding bacterial spores) on inanimate objects. Chemicals used in disinfection are called disinfectants. Sterilization is an absolute condition while disinfection is not.
METHODS OF STERILIZATION: The various methods of sterilization are:1- Physical Methods: a. Radiation method . b. Filtration method . c. Thermal (Heat) methods: I- Dry Heat. II- Moist Heat.2- Chemical Method: By using chemicals.
1- PHYSICAL METHODS OF STERILIZATION: a. Radiation: Since radiation does not generate heat, it is termed "cold sterilization". In some parts of Europe, fruits and vegetables are irradiated to increase their shelf life up to 500 percent. Two types of radiation are used, ionizing and non-ionizing.
I- Non-ionizing rays: Non-ionizing rays are low energy rays with poor penetrative power and with wavelength longer than the visible light. Mechanism of action:1- UV rays inhibits DNA replication.2- Since UV rays don’t kill spores, they are considered to be of use in surface disinfection.
Advantages: UV rays are employed to disinfect hospital wards, operation theatres, virus laboratories, corridors, etc. Disadvantages:1- low penetrative power.2- Organic matter and dust prevents its reach.3- Rays are harmful to skin and eyes.4- It doesnt penetrate glass, paper or plastic.
II- Ionizing rays: ionizing rays are high-energy rays with good penetrative power . These high-energy radiations damage the nucleic acid of the microorganism which kill all bacteria, fungi, viruses and spores. It is used commercially to sterilize disposable petri dishes, plastic syringes, antibiotics, vitamins, hormones, glasswares and fabrics. Examples: X- rays, Gamma rays.
b- Filtration Sterilization : Filtration process does not destroy but removes the microorganisms. Used in the treatment of heat sensitive injections and ophthalmic solutions, biological products and air and other gases for supply to aseptic areas. Application of filtration for sterilization of gases: HEPA (High efficiency particulate air) used in Laminar air flow cabinets.
There are two types of filters used in filtration sterilization:(a) Depth filters: They are made of diatomaceous earth, unglazed porcelain filter, sintered glass or asbestos.(b) Membrane filters: These are porous membrane made of cellulose acetate, polycarbonate or some other synthetic material.Fluids are made to transverse membranes by positive or negative pressure or by centrifugation.
c- Heat: I- Dry Heat: Heat sterilization is the most widely used and reliable method of sterilization, involving destruction of enzymes and other essential cell constituents. Thismethod of sterilization can be applied only to the thermostable products.
1- Red heat:By holding articles in Bunsen flame till they become red hot (bacteriological loops, straight wires)2- Flaming: This is a method of passing the article over a Bunsen flame, but not heating it to redness (scalpels, mouth of test tubes, flasks, glass slides). Even though most vegetative cells are killed, there is no guarantee that spores too would die on such short exposure.
3- HOT AIR OVEN: Used for sterilizing materials that must remain dry e.g. antibiotic powders Hot air circulating ovens are used for this purpose. Articles sterilized: Metallic instruments (like forceps, scalpels, scissors) glasswares (such as petri-dishes, pipettes, flasks, all- glass syringes), and some pharmaceutical products. A temperature of 160-170oC for 2-3 hours is used commonly. Precautions: 1- Articles to be sterilized must be perfectly dry before placing them inside to avoid breakage. 2- The hot air oven must not be opened until the temperature inside has fallen below 60oC to prevent breakage of glasswares.
Advantages:1- It is an effective method of sterilization of heat stable articles. The articles remain dry after sterilization.2- This is the only method of sterilizing oils and powders. Disadvantages:1- Since air is poor conductor of heat, hot air has poor penetration.2- Cotton wool and paper may get slightly charred.3- Glasses may become smoky.4- Takes longer time compared to autoclave.