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. 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 it growing on a medium which suits it, we are 

 said to have obtained a pure culture. The recognition of 

 different species of bacteria depends, in 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. Again, in inquiring as to the possible possession of 

 pathogenic properties by a bacterium, the obtaining of pure 

 cultures is absolutely essential. 



To obtain pure cultures, then, is the first requisite of bacterio- 

 logical research. Now, as bacteria are practically omnipresent, 

 we must first of all have means of destroying all extraneous 

 organisms which may be present in the food media to be used, 

 in the vessels in which the food media are contained, and on all 

 instruments which are to come in contact with our cultures. 

 The technique of this destructive process is called sterilisation. 

 We must therefore study the methods of sterilisation. The 

 growth of bacteria in other than their natural surroundings 

 involves further the preparation of sterile artificial food media, 

 and when we have such media prepared we have still to look 

 at the technique of the separation of micro-organisms from 

 mixtures of these, and the maintaining of pure cultures when the 

 latter have been obtained. We shall here find that different- 

 methods are necessary according as we are dealing with aerobes 



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