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. 
