Sterilization and disinfections By: Microbiology members

Sterilization: is defined as a physical or chemical process that completely destroys all microbial life, including spores. Disinfection: is the process of elimination of most but not necessarily all pathogenic microorganisms (excluding bacterial spores) on inanimate objects. Chemicals used in disinfection are called disinfectants. Sterilization is an absolute condition while disinfection is not (Can be sporostatic but not necessarily sporocidal).

Methods of Sterilization: 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: In this method we use various chemicals.

1- PHYSICAL METHODS OF STERILIZATION: a. Radiation: Since radiation does not generate heat, it is termed "cold sterilization". Two types of radiation are used: 1- Ionizing and, 2- Non-ionizing.

1- UV rays inhibits DNA replication. I- Non-ionizing rays: Non-ionizing ultra violet rays are low energy rays with poor penetrative power. Mechanism of action: 1- UV rays inhibits DNA replication. UV rays have bactericidal effect but don’t kill spores hence they are not efficient for complete sterilization. They can be used in surface disinfection.

Advantages: UV rays are employed to disinfect hospital wards, operation theatres, virus laboratories, corridors, etc. Disadvantages: low penetrative power. Organic matter and dust prevents its reach. It doesn't penetrate glass, paper or plastic. Rays are harmful to skin and eyes.

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. Gamma rays are mainly used for sterilization of prepacked disposable plastic items that can’t withstand heat e.g. plastic syringes, catheters and gloves, and for sterilization of 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, 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 unglazed porcelain filter, 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.

Counting Microorganisms by Filtration

c- Heat: I- Dry Heat: Heat sterilization is the most widely used and reliable method of sterilization. In general, heat kills by denaturing proteins, but membrane damage and enzymatic cleavage of DNA may also be involved. This method 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), 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 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.

LABORATORY HOT AIR OVEN: