CHAPTER IX. 



PHYSIOLOGICAL ACTIVITIES. 



The objects in view in the discussion of the "physiological 

 activities" of bacteria in this and subsequent chapters are 

 to familiarize the student to some extent with the great 

 range of chemical changes brought about by these minute 

 organisms, to show their usefulness, even their necessity, 

 and to impress the fact that it is chiefly by a careful study 

 of these "activities" that individual kinds of bacteria are 

 identified. It should always be borne in mind that the bac- 

 teria, in bringing about these changes which are so charac- 

 teristic in many instances, are simply engaged in their own 

 life struggle, in securing the elements which they need for 

 growth, in liberating energy for vital processes, or occa- 

 sionally in providing conditions which favor their own 

 development and hinder that of their competitors. 



FERMENTATION OF CARBOHYDRATES. 



By this is meant the changes which different carbohy- 

 drates undergo when subjected to bacterial action.' 



These changes are marked chiefly by the production of 

 gas or of acid. ' The former is called "gaseous fermentation" 

 the latter "acid fermentation." The gases commonly pro- 



' The term "fermentation" was originally used to denote the process 

 which goes on in fruit juices or grain extracts when alcohol and gas are formed. 

 Later it was extended to apply to the decomposition of almost any organic 

 substance. In recent years the attempt has been made to give a chemical 

 definition to the word by restricting its use to those changes in which, by 

 virtue of a "wandering" or rearrangement of the carbon atoms "new sub- 

 stances are formed which are not constituents of the original molecule." 

 It may be doubted whether this restriction is justified or necessary. -V 

 definition is at present scarcely possible except when the qualifying adjective 

 is included as "alcoholic fermentation," "ammoniacal fermentation," "lactic 

 acid fermentation," etc. 



