72 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



Organisms may be removed or destroyed, that is, mate- 

 rials may be sterilized, either by physical or by chemical 

 means. The process of chemical sterilization will be dis- 

 cussed later under the heading of action of disinfectants 

 and antiseptics. The physical processes of sterilization are 

 the ones most frequently used in preparation of material 

 for the growth of microorganisms. The physical agencies 

 most important in sterilization are heat, light, and filtra- 

 tion. 



Sterilization by Heat. — Bacteria, yeasts, and molds, like 

 all higher forms of life, are killed if they are subjected to a 

 temperature sufficiently high, and for a sufficient length of 

 time. To sterilize by heat implies subjecting the material 

 to be sterilized to such a temperature and for such a period 

 of time as will quite certainly destroy all microorganisms 

 which are present. The methods commonly used are ex- 

 posure to the direct flame, to hot air, to streaming steam, 

 and to steam under pressure. 



Platinum needles and other small metallic (or occasion- 

 ally glass) objects used in the laboratory are usually steril- 

 ized by placing them directly in the Bunsen flame. A 

 needle, for example, may be quickly raised to a red or 

 white heat. Momentary exposure, under these conditions 

 will, of course, destroy all organisms which are present. 

 Incineration is also the method most commonly used in the 

 disposal of the bodies of animals that have died of infec- 

 tious diseases, and for the destruction of materials of little 

 value which have been contaminated with disease-producing 

 microorganisms. 



Dry heat is commonly used for sterilization of the labora- 

 tory glassware such as test tubes, flasks, and pipettes. The 

 so-called hot air oven is ordinarily used for this purpose. 

 Materials to be sterilized are subjected to a temperature of 

 165° to 175° C. for an hour or more. The long exposure is 

 in part necessary to make sure that the heat has penetrated 



