CHAPTER VII. 



CULTURE-MEDIA AND THE CULTIVATION OF 

 MICRO-ORGANISMS. 



IN order to observe them accurately the organisms must be 

 separated from their natural surroundings and artificially 

 cultivated upon certain prepared media of standard compo- 

 sition, in such a manner that only organisms of the same 

 kind are together. The effects of one organism upon the 

 growth of another, by neutralizing its metabolic products, 

 by changing the reaction of the medium in which it grows so 

 as to inhibit further multiplication, by dissolving the other 

 species through its enzymes, etc., suffice to show how im- 

 possible it is to determine the natural history of any organ- 

 ism unless it be kept strictly away from other species. 



Fortunately the same general principles apply equally for 

 the cultivation of all forms of micro-organismal life, and much 

 the same media apply in all cases. What is said, therefore, 

 about the bacteria may be regarded as appropriate for all. 



Various organic and inorganic mixtures have been sug- 

 gested for the cultivation of bacteria, but few have met with 

 particular favor and become standards. At the present 

 time certain standard media are used in every laboratory 

 in the world; all systematic study of the organisms depends 

 upon the behavior of bacteria upon them, and no study 

 of micro-organisms can be regarded as complete unless the 

 behavior of the bacteria upon them has been carefully 

 considered. 



Our studies of the biology of the bacteria have shown that 

 they grow best in mixtures containing at least 80 per cent, of 

 water, of neutral or feebly alkaline reaction, and of a com- 

 position which, for the pathogenic forms at least, should 

 approximate the juices of the animal body. It might be 

 added that transparency is a very desirable quality, and that 

 the most generally useful culture media are those that can 

 be liquefied and solidified at will. 



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