122 BACTERIA, YEASTS, AND MOLDS 



water. They can be dried without injury, may be frozen! 

 for months without losing their vitality, and even a short 

 boiling fails to kill many species. With all these wonder- 

 ful properties it is not strange that they are constantly at 

 work all around us modifying the nature of all substances] 

 upon which they feed. 



Parasites and Saprophytes 



The food upon which bacteria feed may be either living 

 or dead. If bacteria are capable of feeding upon the 

 living body of an animal or plant, we call them p arasites . 

 Such bacteria quite naturally produce injury to the life 

 of the individual upon which they feed. In mankind they 

 produce a great variety of abnormal results which we call 

 diseases. The parasitic bacteria, therefore, are commonly 

 called disease germs, and are the cause of most of our con- 

 tagious diseases. Many of them feed upon animals, pro- 

 ducing animal diseases, while others live upon plants, 

 giving rise to diseases in the plant world. 



Fortunately only a comparatively small number of spe- 

 cies of bacteria are capable of existing upon living bodies 

 of animals. The great majority are incapable of feeding 

 upon living tissue, although they feed upon it readily 

 enough after it is dead. Those dependent upon the dead 

 bodies of animals or plants cannot live a parasitic life. 

 When bacteria feed upon such nonliving materials we call 

 them saprophytes. They are, like animals in general, depend- 

 ent for sustenance upon dead animal and vegetable food. 

 The saprophytic bacteria, while they may be rivals of ani- 

 mals for food, are not the cause of diseases. These harmless 



