CHAPTER IX. 

 PHYSIOLOGICAL ACTIVITIES. 



The physiological activities of motion, reproduction and 

 metabolism within the cell have been discussed in previous 

 chapters. 



The objects in view in the discussion of such physio- 

 logical activities as are treated 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 care- 

 ful study of these "activities" that individual kinds of 

 bacteria are identified. It should always be borne in mind 

 that the bacteria, in bringing about these changes which are 

 so characteristic 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 

 occasionally in providing conditions which favor their own 

 development and hinder that of their competitors. Most 

 of these changes are incidental to, are by-products of, this 

 life struggle. 



Those physiological activities discussed below in which 

 chemical transformations are the pronounced features are 

 frequently termed "biochemical" activities. 



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 acid. The former is called "gaseous fermentation" 

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



1 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 

 (78) 



