CHAPTER IV. 



STERILIZATION AND DISINFECTION. 



Sterilization of Culture-media and Laboratory Apparatus. 



BECAUSE of the presence of bacteria in the air and in 

 rooms, and on articles of furniture, etc., a very important 

 part of bacteriologic technique consists in destroying these 

 micro-organisms by sterilization and disinfection. In this 

 way the contamination of the culture-media is prevented and 

 germs can be studied in pure culture, free from all other 

 organisms. 



Sterilization is accomplished either by heat, filtration, or the 

 action of chemicals. Usually the term sterilization is intended 

 to imply destruction of bacteria by heat. Disinfection means 

 the destruction of bacteria by the use of chemicals. Any 

 substance which is capable of killing bacteria is called a 

 germicide. One which inhibits the development of bacteria 

 is called an antiseptic. An object is said to be sterile when it 

 is entirely free from bacteria and their spores. An object is 

 septic when it contains actively growing bacteria or their 

 poisonous products. Aseptic is synonymous with sterile. 



All bacteria have a thermal death-point, and the method of 

 sterilizing and time of exposure are regulated accordingly. 

 Culture-media, fluids, and anything that can be subjected to 

 it, are sterilized by some form of heat. 



STERILIZATION BY HEAT : This is accomplished by fire ; 

 dry heat or hot air ; live steam ; superheated steam, or steam 

 under pressure ; and boiling. 



Fire : This form of sterilization is absolutely certain in its 

 results, because it completely destroys all infected matter. 

 Naturally that would limit its use considerably. In the labo- 

 ratory the scissors, knives, forceps, and inoculating needles 

 are sterilized by passing them through the flame of a Bunsen 



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