CHAPTER VII. 

 CHEMICAL ENVIRONMENT. 



REACTION OF MEDIUM. 



Most bacteria are very susceptible to changes in the 

 degree of acidity or alkaUnity of the medium in which they 

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

 slightly alkaline, and some a neutral (with reference to 

 litmus as indicator) . 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 bac- 

 teria and as a consequence produce poor crops. The disease- 

 producing bacteria as a class grow best in a medium which 

 is slightly alkaline. 



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 Alaking of ]\Iedia, 

 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 

 kind and amount of food material a^■ailable — a rich protein 



