256 BACTERIOLOGY. 



the parasitic form of a pleomorphic microbe, trie 

 other forms of which make their appearance 

 only in the course of its saprophytic existence 

 and were hence at first entirely overlooked. 

 The group of facultative parasites comprises 

 those species which can maintain and repro- 

 duce themselves in a purely saprophytic way 

 upon lifeless material without ever necessarily 

 attacking living hosts as parasites ; indeed to 

 attain certain stages of development it is neces- 

 sary that they should live the life of real 

 saprophytes.* To this latter group belong the 

 majority of the disease-producing bacteria now 

 known, such as the bacteria of anthrax, typhoid 

 fever, and cholera. 



Finally, there are bacteria which in a strict 

 sense never invade the living organism, but 

 yet are dangerous and able to provoke disease. 

 Many of the bacteria of putrefaction are able 

 to generate out of lifeless nutrient substances 

 poisons that can act injuriously on human be- 

 ings without participation of the poison-forming 

 bacteria themselves. This may even happen in 

 the normal organism during intestinal putre- 

 faction, a process which seems in itself to have 

 become necessary as a result of adaptation. 

 Such bacteria may be designated as ceco-para- 

 sites, and may be regarded as forms on the 

 road to become facultative parasites. 



