CHAPTER VII. 



CULTURES AND THEIR STUDY. 



A culture is an artificial growth of bacteria on one or 

 another specially prepared medium. Any number of different 

 kinds of bacteria may be contained in a culture. If the 

 growth contains only one variety, it is called a pure culture. 

 The objects in view in the preparation of culture-media are 

 the separation of bacteria and the making of a pure culture, 

 so that the morphology and biology of each variety of organ- 

 ism may be studied accurately. The disease caused by any 

 one germ may be reproduced experimentally in the labora- 

 tory by the injection of a pure culture of the germ in ques- 

 tion. Until Koch suggested the use of solid culture-media 

 and the making of plate cultures it was impossible to isolate 

 bacteria and make pure cultures of them. Microscopic 

 examination alone is frequently insufficient for a positive 

 identification of a germ, as many species have the same 

 morphologic characteristics. Hence it is a matter of neces- 

 sity to cultivate these germs artificially, so that their cultural 

 differences may be observed. 



Bacteria are isolated in pure culture by the Koch plate 

 method ; Petri dish method ; or Esmarch roll culture. 



The preliminary steps for making a pure culture by any of 

 these methods are as follows : Label three tubes of gelatin or 

 agar-agar 1, 2, 3 ; liquefy the medium over a flame or in 

 water kept at a temperature of about 40 C. Sterilize the 

 platinum needle (Fig. 20) and inoculate tube No. 1 with a 

 loopful of the material to be examined. Replace the plug 

 and shake the tube gently, being careful not to wet the cotton 

 plug. Inoculate tube No. 2 in the same manner with a few 

 loopfuls obtained from tube No. 1 ; tube No. 3 is inoculated 

 from tube No. 2. This gives us three different dilutions. 

 Another method for obtaining a pure culture is to inoculate 



5 Bact. 65 



