BIOLOGY Volume 2 - A Guide to General Biology - 2004

15. HEALTH AND DISEASE

15.4. Disinfectants, sterilization and antisepsis

15.4.2. Sterilization

Sterilization is defined as the elimination of all forms of life from inanimate objects. For instance, it is often necessary to ensure the complete absence of microbes in Water used for certain chemical processes or in liquid nutrient media prepared for growing pure microbiological cultures. Solid objects (such as surgical instruments and garments) and entire rooms (such as operating theatres) are subjected to sterilization. The following Methods are used for this purpose.

1. Heat Treatment. People have known since ancient times that heating food can prevent spoilage during storage. The heat treatment regime (its duration and Temperature) depends on the type of product and the specific microbes that need to be eliminated. For example, to destroy tuberculosis pathogens in milk, 'pasteurization' is sufficient, i.e., incubation at 70 °C for 15—30 minutes (Section 15.3.2).

If heat treatment is applied to hermetically sealed food (such as canned goods), it can be stored for a very long time. Spores of most Bacteria are killed at a temperature of 120 °C. A notable exception is the bacterium Clostridium botulinum. To destroy the spores of this bacterium, which causes botulinum poisoning, heating the product at 250 °C for approximately 200 minutes is required. If the heat treatment regime is not followed and the spores survive, the anaerobic (oxygen-independent) microorganism Clostridium botulinum can multiply inside the can, releasing a lethal toxin.

2. Steam treatment. This is carried out in an autoclave—a steel chamber with a door for loading and unloading the objects to be sterilized. Steam is pumped into the chamber. Treatment time depends on its temperature and the pressure inside the autoclave. For instance, adequate sterilization at 120° C and a pressure of 150 kN/m² takes only 12 minutes.

3. Irradiation. Visible light has no adverse effect on biological objects; however, radiation with a shorter wavelength, such as ultraviolet light, can damage Multicellular Organisms and kills bacteria. X-rays and gamma rays are even more effective. They make it easy to sterilize disposable syringes, scalpels, dressings, and certain food products under easily controlled conditions.