CHAPTER VII 



CULTURE-MEDIA AND THE CULTIVATION OF MICRO- 

 ORGANISMS 



In order to observe them accurately micro-organisms must be 

 separated from their natural surroundings and artificially cul- 

 tivated upon certain prepared media of standard composition. The 

 effects of one organism upon the growth of another, by neutrahzing 

 its metabolic products, by changing the reaction of the medium in 

 which it grows so as to inhibit further multipUcation, by dissolving 

 the other species through its enzymes, etc., suffice to show how 

 impossible it is to determine the natural history of any organism 

 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 aU cases. What is said, therefore, about the bacteria 

 may be regarded as appropriate for all. 



BACTERIA 



Various organic and inorganic mixtures have been suggested for 

 the cultivation of bacteria, but few have met with particular favor 

 and become standards. At the present time a few well-known 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 

 behavior 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 composition which, for 

 the pathogenic forms at least, should approximate the juices of the 

 animal body. It might be added that transparency is a very desir- 

 able quality, and that the most generally useful culture-media are 

 those that can be hquefied and solidified at will. 



All accurate bacteriologic culture experiments require that an 

 exact knowledge of the chemistry of the media used shall be at hand. 

 The importance of this knowledge is suggested by the pains taken 

 to arrive at it. The best bacteriologists of America have agreed 

 upon certain details that are explained in the following excerpts 

 from the Report of the Committee of Bacteriologists of the American 

 Public Health Association.* 



* "Jour. Amer. Public Health Assoc," Jan., 1898, p. 72. 

 189 



