CHAPTER VII. 

 CHEMICAL ENVIRONMENT. 



REACTION OF MEDIUM.i 



Most bacteria are very susceptible to changes in the 

 degree of acidity or alkaHnity of the medium in which they 

 grow. Some kinds prefer a shghtly acid reaction, some a 

 shghtly alkaline and some a neutral. The organism which 

 is the commonest cause of the souring of milk thrives so 

 well in the acid medium it produces that it crowds out 

 practically all other kinds, though its own growth is 

 eventually stopped by too much acid. Acid soils are 

 usually low in numbers of bacteria and as a consequence 

 produce poor crops. The disease-producing bacteria as a 

 class grow best in a medium which is near the neutral point. 



Accurate determination of limits have been made on but 

 few organisms. The reaction is a most important factor in 

 growing bacteria on artificial media (see Making of Media, 

 Chapter XVI). 



INJURIOUS CHEMICAL SUBSTANCES. 

 (See Disinfection and Disinfectants, Chapter XIII.) 



CHEMICAL COMPOSITION. 



The chemical composition is subject to wide variation 

 chiefly for two reasons: First, the cell wall in most instances 

 seems to exert only a slight selective action in the absorption 

 of mineral salts, so that their concentration within the cell 

 is very nearly that of the surrounding medium. Second, 

 the chief organic constituents vary remarkably with the 



1 For a discussion of reaction see page 161. 



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