CHAPTER VI 



CULTURAL METHODS 



Whenever practicable it is customary to study cultures 

 of bacteria wben grown under laboratory conditions. Many 

 species of bacteria, as they occur in nature, are usually 

 found growing together, that is, in mixed cultures. They 

 are widely distributed, making it somewhat difficult to 

 maintain the bacteria in pure cultures after they are once 

 obtained. For an adequate laboratory study of cultural 

 characters it is necessary first to sterilize suitable contain- 

 ers, usually glassware, second to prepare a suitable medium 

 or nutrient mixture upon which the organisms will grow, 

 third to secure the organism to be studied tn pure culture, 

 that is, free from admixture with all other kinds of bacteria, 

 and fourth to study the characteristic growth and appear- 

 ance upon the culture media. 



Methods of Sterilization 



Sterilization may be defined as that process whereby 

 materials may be rendered entirely free from living micro- 

 organisms. Bacteria are so widely distributed in nature 

 that it is usually taken for granted that they are present in 

 or upon practically all the materials which are used in the 

 laboratory. Practically the only source of material natu- 

 rally sterile is from the normal tissues of animals or plants. 

 It is, for example, possible so to bleed an animal as to secure 

 sterile blood serum without any process of sterilization. In 

 most cases some artificial method of removing the bacteria 



is used. 



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