CHAPTEE 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 got it to grow 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 

 bacteriological 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 



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