GENERAL CHARACTERS OF BACTERIA. 21 



these different forms subsist is by no means exact, for each group 

 may contain members that make use of all kinds of foods; but, 

 generally speaking, the higher molds and mushrooms attack the 

 harder plant tissues, the yeasts attack sugars, while the bacteria are 

 especially concerned in the destruction of proteids. 



The food which bacteria consume may be either living or dead 

 when they attack it. In the case of most bacteria the organisms 

 are unable to feed upon the material while it is alive. If bacteria, 

 for example, are placed upon living muscle, they are usually unable 

 to attack it and soon die; but if they are placed upon the same 

 muscle after it is dead, they feed upon it readily and cause it to 

 putrefy. Those organisms which feed upon lifeless bodies include 

 the vast majority of bacteria. There are, however, other species 

 that are capable of living upon the bodies of animals and plants 

 while these are still alive. Inasmuch as they can feed upon living 

 organisms they are liable to produce disease and constitute in general 

 the disease bacteria. Bacteria feeding upon living animals and 

 plants are called parasites. Bacteria feeding upon dead animals 

 and plants are called saprophytes. Both saprophytes and parasites 

 are of great importance. 



