66 BACTERIOLOGY. 



Saprophytic and Parasitic Bacteria. 



Bacteria are grouped, according- to their habitat, into 

 saprophytic and parasitic. The saprophytic bacteria are 

 those which live on dead animal, or vegetable matter. The 

 vast majority of bacteria belong- to this group. Only a 

 relatively small number of organisms possess the power of 

 growing in the living animal or plant. These are, there- 

 fore, designated as parasitic bacteria. Because the latter 

 grow and thrive in the living body it must not be inferred 

 that they subsist entirely on living matter. The waste 

 products of the living, or of the dead cells may furnish the 

 necessary food supply. The parasitic bacteria include 

 many harmless forms in addition to the disease -producing 

 organisms. 



The majority of the bacteria present in the mouth, stom- 

 ach and intestines cannot be regarded as parasitic. They 

 live on the dead matter in the alimentary tract and can live 

 equally well outside of the body. They are to be consid 

 ered as saprophytic bacteria. A few of the mouth bacteria, 

 such as the vibrios and spirals, cannot be made to grow 

 on artificial media. They are, apparently, dependant upon 

 the living organism. This is equally true of the leprosy 

 bacillus and to a certain extent of the germs of gonorrhea 

 and of tuberculosis. It is customary to designate those 

 bacteria which are compelled to live in the living body as 

 obligative parasites. 



On the] other hand, among the saprophytic bacteria 

 there are those which are unable to thrive in the living 

 body. They are compelled, as it were, to live on the dead 

 matter found in nature. Consequently, they are known as 

 obligative saprophytes. It is evident that the obligative 

 parasites and the obligative saprophytes constitute two 

 extreme groups. Numerous bacteria occupy an intermedi- 



