CHAPTER LIII, 



BACTERIA: THEIR WORK. 



I. The food which is needed for the nutrition of nearly all 

 bacteria is either derived by the latter from dead organic 

 substances or from living organisms. Those bacteria which 

 feed upon dead organic matter are termed saprophytes, those 

 which live upon the organic compounds within the tissues of 

 living animals or plants being designated parasites. Many 

 parasitic bacteria are the cause of infectious diseases of which 

 tuberculosis, diphtheria, and typhoid fever are typical examples. 

 Perhaps the saprophytic species claim the greatest share of 

 the farmer's attention, for they are the special agents concerned 

 in the processes of putrefaction and decay by which the dead 

 bodies of animals and plants are ultimately converted into 

 simple substances such as ammonia, nitrates, carbon dioxide and 

 water, which are of paramount importance in the nutrition of 

 green plants. Moreover, to this class of bacteria we owe a 

 number of useful, as well as baneful, chemical changes such 

 as occur in the souring of milk and beer, the ripening of cheese, 

 the nitrification of manures, and many other familiar decom- 

 positions which organic substances undergo. 



These chemical changes are usually included under the 

 term fermentation and are connected in some unexplained 

 manner with the vitality and multiplication of certain bacteria, 

 for they do not take place unless these specific organisms are 

 present and in a living condition. Most of these fermenta- 

 tions are complicated physiological processes which cannot at 

 present be expressed by chemical equations although the 



