CHAPTER II 



THE STUDY OF THE CULTURAL CHARACTERS AND 

 BIOCHEMICAL FUNCTIONS OF BACTERIA 



It is important that students shall study any new or old bac- 

 terial forms over a sufficiently long period to fully establish their 

 characters. To publish a description of a bacterium after a study 

 of one or two generations is the height of superficiality. An 

 organism should be observed over a considerable period of time, 

 at least until its characters become fixed and constant. Forms 

 freshly isolated from a natural habitat, as soil or water, fre- 

 quently show certain modifications of their characters after suc- 

 cessive cultivation on artificial media; so much so, that a 

 description of a species in its early generations may differ rather 

 widely from those of far later periods. Eventually the organism, 

 conforming to its new environment, will establish characters 

 which are reasonably constant. These variations, should they 

 occur, need to be embodied in a description, or such a sufficient 

 range given to the descriptive characters as to include said cul- 

 tural variations. It is a familiar fact that slight differences in 

 the condition of the medium will modify the macroscopic char- 

 acters of a growth. Thus, in milk cultures, it may take very 

 little to disturb the balance between an unchanged appearance of 

 the medium and the formation of a coagulum. Indol formation 

 may also become an uncertain factor in the biochemistry of an 

 organism. Gelatin colonies in their microscopic characters are 

 open to wide variations, so much so that it is a question whether 

 they have any great value in species differentiation. In fact, 

 even the macroscopic appearance of a gelatin colony may be 



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