CHAPTER II. 

 METHODS OF CULTIVATION OF BACTERIA. 



Introductory. In order to study the characters of any 

 species of bacterium it is necessary to have it growing apart 

 from every other species. In the great majority of cases 

 where bacteria occur in nature, this condition is not fulfilled. 

 In the general processes of putrefaction many different 

 species occur all mingled with each other. Only in the 

 blood and tissues in some diseases do particular species 

 occur singly and alone. We usually have, therefore, to 

 remove a bacterium from its natural surroundings and grow 

 it on an artificial food medium. When we have succeeded 

 in separating it, and have got it to grow on a medium which 

 suits it, we are said to have obtained a pure culture. These 

 pure cultures are absolutely necessary for the proper study 

 of bacteria ; for, when many individuals of a species are 

 growing together, the mass formed by their aggregation 

 frequently presents characteristic appearances which con- 

 stitute specific differences. The recognition of different 

 species of bacteria depends, m fact, far more on the 

 characters presented by pure cultures and their behaviour 

 in different food media, than on microscopic examination. 

 The latter in most cases only enables us to refer a given 

 bacterium to its class. For the greater number of specific 

 characters we rely on the observation of pure cultures. 

 Again, in inquiring as to the possible possession of patho- 

 genic properties by a bacterium, the obtaining of pure 



