THE ENVIRONMENT OF BACTERIA. 67 



ate position between these extremes. There are bacteria 

 which may primarily be considered as saprophytic, but 

 which, at times, may lead a temporary, parasitic existence. 

 These are designated as facultative parasitic bacteria. 

 Again, certain bacteria which are primarily parasitic may 

 be able to thrive somewhat, outside of the body, on dead 

 matter. They elect, so to speak, a saprophytic life and 

 hence are known as facultative saprophytes. 



The transition from one extreme to the other can per- 

 haps be seen best from the following' arrangement of the 

 several groups: 



Obligative saprophytic bacteria, 

 Facultative parasitic bacteria, 

 Facultative saprophytic bacteria, 

 Obligative parasitic bacteria. 



The parasitic as well as saprophytic bacteria are 

 unable to utilize carbonic acid as a food. They cannot 

 build up protoplasm out of wholly inorganic substances. 

 In other words, they are dependent upon organic compounds 

 which were made by animal or plant life. The group of 

 nitro-bacteria (p. 61) constitutes an exception to this rule, 

 inasmuch as these can exist on wholly inorganic matter. 

 Consequently they do .not, strictly speaking, belong in the 

 group of saprophytic bacteria. 



The parasitic bacteria, as mentioned above, include the 

 various disease-producing organisms. The saprophytic 

 bacteria include those forms which thrive on dead matter, 

 and consequently the bacteria which produce fermentation 

 and putrefaction fall under this head. 



