CHAPTER VII. 



Parasites and Saprophytes. 



Putrefactive Bacteria — Pigment-forming Bacteria — Enzyme-forming Bacteria 

 — Non-pathogenic, Saprophytic, Parasitic, Pathogenic Bacteria — 

 Sarcina Ventriculi — Leptothrix Buccalis — Facultative Saprophytes — 

 Facultative Pathogenic Parasites — Anthrax Bacillus — Pathogenic, 

 Parasitic, Saprophytic, merely Relative Terms. 



In order that rauch of what occurs in the following chapters 

 may be understood, it is necessary that some idea should be 

 given of the way in which the terms saprophytic, parasitic, 

 and pathogenic are used. The first of these is not so impor- 

 tant ; the term saprophytic as applied to bacteria is being 

 gradually supplanted by the term non-pathogenic, especially 

 in works dealing with the relation of micro-organisms to 

 disease. Speaking generally, saprophytic bacteria are those 

 bacteria that have the power of obtaining the nutriment 

 they require for their building up, and for the carrying on of 

 their vital functions, from dead vegetable or animal substances 

 only. They are not able to invade the tissues of living 

 animals and plants, but they play an important part, as we 

 have already seen, in bringing about oxidation of various 

 organic materials and of giving rise to certain definite com- 

 pounds. To this class belong the putrefactive bacteria, and 

 many of the pigment-forming bacteria ; the yeasts or fermen- 

 tation fungi are also to be looked upon as purely saprophytic 

 in character ; and such organisms as the Bacillus amylo- 

 bacter, the lactic acid bacillus, and the like, are all probably 

 to be classified in this group. 



The subdivision of the group has been attempted by 

 many workers, and the saprophytic organisms have been 

 arranged according to their power of action on the various 

 carbo-hydrates and on proteid substances. Others have 

 taken their products as a basis on which to classify 

 them, and enzyme-forming, pigment-forming, or alcohol- 



II 



