CHAPTER VII 



THE PREPARATION OF CULTURE MEDIA 



GENERAL TECHNIQUE 



THE successful cultivation of bacteria upon artificial media re- 

 quires the establishment of an environment which shall be suitable 

 in regard to the presence of assimilable nutritive material, moisture, 

 and osmotic relations. These requirements are fulfilled in the com- 

 position of the nutrient media described in another section, media 

 which are to some extent varied according to the special require- 

 ments of the bacteria which are to be cultivated. If cultivation, 

 furthermore, is to have any value for scientific study of individual 

 species, it is necessary to obtain these species free from other varie- 

 ties of microorganisms, that is, in pure culture, and to protect such 

 cultures continuously from contamination with the other innumer- 

 able species which are everywhere present. 



The technique which is employed for these purposes has been 

 gradually evolved from the methods originally devised by Pasteur, 

 Koch, Cohn, and others. 



Bacterial cultivation is carried out in glassware of varied con- 

 struction, the forms most commonly employed being test tubes of 

 various sizes, Erlenmeyer flasks, the common Florence flasks, and 

 Petri dishes. All glassware, of course, must be thoroughly cleansed 

 before being used. 



Preparation of Glassware. The cleansing of glassware may be 

 accomplished by any one of a number of methods. New glassware 

 may be immersed in a one per cent solution of hydrochloric or nitric 

 acid in order to remove the free alkali which is occasionally present 

 on such glass. It is then transferred to a one per cent sodium 

 hydrate solution for a few hours, and following this is washed in 

 hot running water. 



In the case of old glassware which has contained culture media, 

 sterilization in the autoclave is first carried out, then the glassware 

 is boiled in five per cent soda solution or in soapsuds. After this, 



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