METHODS OF SECURING PURE CULTURES OF BACTERIA 109 



from time to time. The organisms are separated from each 

 other by this process of dilution, and are held fast by the solidifica- 

 tion of the medium. In most cases the conditions are favorable 

 for growth, and development begins. Within a few days sufficient 

 multiplication takes place, so that the mass of organisms that has 

 developed from the single isolated individuals has reached a size 

 that can be easily seen with the unaided eye. Such a mass of organ- 

 isms is termed a colony. Transfers from such colonies will show 

 only a single kind of organism present, and by making isolations 

 from each type of colony, pure cultures may be secured of each 

 species present. 



Isolation by the Use of Heat. When it is desired to isolate a 

 spore-producing organism from non-sporulating forms, the culture 

 may be heated to 80 for fifteen minutes. This will not destroy 

 the spores, but will eliminate all other cells. If one species of 

 spore-forming organism only is present, this results in a pure cul- 

 ture at once ; if more than one species, plating becomes necessary. 



Isolation by the Use of Differential Antiseptics or Disinfect- 

 ants. Not all species of bacteria are affected alike by a given 

 antiseptic or disinfectant, and it is sometimes possible to add a 

 substance that will prevent the growth or kill one form without 

 interfering seriously with the growth of others. A small amount 

 of phenol added to bouillon will inhibit the growth of most bacteria, 

 with the exception of certain members of the intestinal group. 

 A still better example of such substance is antiformin, which, 

 when mixed with sputum or other materials containing tubercle 

 bacilli, destroys all other organisms than these, and enables one to 

 secure a pure culture at once. This will be discussed in greater 

 detail under the heading of Tuberculosis. 



Isolation by Animal Inoculation. Some species of pathogenic 

 bacteria develop very slowly upon artificial media, or require 

 a special medium for their growth. When these occur mixed 

 with other organisms, it is sometimes difficult to secure them in 

 pure' culture. This difficulty may in some cases be overcome by 

 animal inoculation. The injection of the organism into a suitable 

 susceptible animal results in thedestruction-bythe body of the other 

 bacteria injected at the same time, and the characteristic organism 

 may later be isolated in pure cultures from the lesions of the disease. 



