intensity of the application. Temperature in a refrigerator ranges from 0-8"C and has a bacteriostatic effect which reduces the metabolic rate of most organisms so that they cannot reproduce or synthesize toxins. Freezing at -20"C kills most bacteria, but some may survive in a frozen state. 3. drying or desiccation To grow and multiple, microorganisms require water. The removal of water by evaporation or freeze- drying (solid to gas) inhibits growth and reproduction of microorganisms by inhibiting enzymes. They may be viable for years so when water is made available they resume growth and reproduction. ultra-violet radiation and ionizing radiation (x-rays and ~gamma rays) Both types of radiation damage the DNA of microorganisms and denatures their proteins. U-V radiation cannot penetrate materials such as glass, clothing, dirt, paper, or pus and is used to kill microorganisms on surfaces. Germicidal lamps in operating rooms, nurseries, and communicable disease wards reduce the number of bacteria in the air, but they do not sterilize 5. filtration - Filtration is the passage of material or liquids through a filter containing small pores that retain microorganisms. Membrane filters of cellulose acetate are the most common, but gauze, cotton, and paper serve as filters for air. Vaccines that require the presence of live viruses, such as polio, are passed through filters which prevent bacteria from going through the pores. Air filters are used in operating rooms to lower the number of airborne microorganisms
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