BIOLOGIC AND CHEMIC ACTIVITIES 27 



few requirements for their existence. In a drop of water 

 1,700,000,000 cocci can find space. 



Very few places are free from germs ; the air on the high seas 

 and on the mountain-tops is said to be free from bacteria, 

 but this is questionable. 



Specific Nature. One kind of bacterium will not pro- 

 duce another kind. A bacillus does not arise from a micro- 

 coccus, or the typhoid fever bacillus produce the bacillus of 

 tetanus. 



Saprophytes and Parasites. Saprophytes: aairpos, put- 

 rid; <f>VTOv, plant. Parasites: Trapa, aside of; crlros, food. 

 Those bacteria which live on the dead remains of organic life 

 are known as saprophytic bacteria, and those which choose the 

 living bodies of their fellow-creatures for their habitat are 

 called parasitic bacteria. Some, however, develop equally 

 well as saprophytes and parasites. They are called faculta- 

 tive parasites. All pathogenic (disease-producing) bacteria 

 are parasites. 



Conditions of Life and Growth of Bacteria. Influence 

 of Temperature. In general, a temperature ranging from 10 

 C. to 40 C. is necessary to the life and growth of bacteria. 



Saprophytes take the lower temperatures; parasites, the 

 temperature more nearly approaching the animal heat of the 

 warm blooded. Some forms require a nearly constant heat, 

 growing within very small limits, as the bacillus of tubercu- 

 losis. 



Some forms can be arrested in their development by a 

 warmer or colder temperature, and then restored to activity 

 by a return to the natural heat. 



A few varieties exist only at freezing-point of water, and 

 others again will not live under a temperature of 60 C. and 

 thrive in hot springs at a temperature of 89 C. 



For the majority of bacteria a temperature of 60 C. will 

 prevent development, but steam under pressure at 125 C. is 

 necessary to destroy spores. Ice may contain active bac- 

 teria; frozen milk permits the growth of bacteria. 



Influence of Oxygen. Two varieties of bacteria in relation 



