Methods of Sterilization

Methods of Sterilization

	Sterilisation Sterilization is the complete destruction or removal of all living organisms from the object being sterilised. The development of methods of sterilization was mainly a consequence of the controversy over spontaneous generation culminating in the work of Pasteur. Experiments designed to prove or to disprove spontaneous generation depended upon two general principles: (I) the complete sterilisation of a suitable growth medium so that no living organisms exist at the start of the experiment, (2) the design of the vessel of a type that it is impossible for microbes to enter from outside.
This was necessary following the realisation of the existence of microbes floating around in the air If these two principles arc strictly followed and conditions are otherwise suitable for multiplication of microbes, any growth occurring must be the result of spontaneous generation. Thus, key question was how good methods were for (i) attaining, and (ii) maintaining sterility.
Let us consider these two principles. (i) The attainment of sterility The usual method depended upon heat treatment. However, it was soon realised that microorganisms vary widely in their resistance to heating. Bacteria require temperature and some also produce heat stable spores. Thus boiling at normal pressure was insufficient to kill these spores and, therefore, autoclave was designed to increase the pressure, and, thereby, the temperature. sterilisation. Sealing of the flask was not proper as oxygen, known to be essential for many forms of life, could no longer enter the vessel. It was necessary, therefore, to include some kind of filter to prevent the entry of microbes but not of air. This led to the development of the cotton wool plug, that was soon adopted universally by microbiologists. By the end of the 19th century most of the methods currently used for sterilisation had been developed.
These are briefly summarised below: 1. Heat. For general sterilisation a time and temperature that kill all organisms including heat-resistant spores is used. The methods generally adopted are as follows: ( a) Wet heat in an autoclave. The usual met/1od is a time of 30 minutes at a pressure of 1.05 kg/cm2 that will give a temperature of 121°C. This is the best method, if practicable. (b) Tyndallization. This is a course of three periods of boiling at 100°C for 30 min. at daily intervals. (c) Dry heat. This is done in a dry oven, where a temperature of 160° C for two hours is usually required. 2. Filtration. The liquid or gas to be sterilised is passed through a filter with a porosity sufficient to remove any microorganism in suspension. The cotton wool is used for gases. For liquids, a variety of(filters are available, made of materials such as cellulose nitrate (millipore filters). This method is very useful for sterilisation of liquids containing heat-labile components.
	3. Radiation. Ultraviolet light is very effective in sterilisation of air. For solids, we generally use gamma rays or x-rays from a source such as radioactive cobalt. Ionizing radiation is often used to sterilize plastics and other heat labile materials. 4. Chemicals. Many chemicals are lethal to microbes. Hypochlorite solution and phenolic derivatives are used as general laboratory disinfectants. Similar chemical is gaseous ethylene oxide. However, these may not cause sterilisation under some conditions.

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