Cultivation and Identification of Bacteria 125 



medium. Not all such isolated organisms survive, but if growth 

 occurs in the broth, the offspring can all be traced back to a single 

 cell, and studies relative to nutrition and genetics will have more 

 significance when they are based upon cultures originating from 

 known single cells. 



SELECTIVE HEATING 

 It has been pointed out that spores resist higher temperatures 

 for longer periods of time than do vegetable cells. To separate a 

 spore-former from a mixed culture, the culture is subjected to 

 varying degrees of heating, and the surviving spores can then be 

 separated. Not all spores are equally resistant to heat, so heating 

 of parts of the suspension at different temperatures may be neces- 

 sary to separate mixtures of spore-formers. 



USE OF LABORATORY ANIMALS 

 Some animals are known to be extremely susceptible to the ac- 

 tion of specific organisms. The mouse, for example, can be in- 

 jected with a mixture of many organisms, but if in that mixture is 

 found a virulent pneumococcus, it is quite possible for the natural 

 destructive forces of the mouse to dispose of all of the injected 

 bacteria except the pneumococcus. This organism and its progeny 

 mav eventually destroy the mouse, oftentimes in less than 24 hours. 

 Other organisms, however, are also capable of being pathogenic 

 for mice. An examination of the peritoneal cavity of the mouse 

 immediately after death will frequently reveal a pure culture of 

 the pneumococcus. The guinea pig serves as a similar filter for 

 separating the tuberculosis microbes from materials such as sputum. 

 This purification process may take from 4 to 6 weeks, however, 

 because of the slow metabolism of the tuberculosis organisms. 

 Autopsy examination of the lungs, liver, spleen, and other organs 

 will generally reveal pure cultures of packed organisms in tubercles 

 —visible growths of the pathogenic agent. 



When Koch's postulates were discussed it was made clear that 

 unless a susceptible animal is used, it is not possible to prove the 

 etiology of all diseases by his postulates. This is true for some 



