22 THE GENERAL CHARACTERS OF MICROORGANISMS 
be fed with organic food. In other words, they require for their 
life the same kind of foods that the animal kingdom requires. 
They can consume proteids, starches, sugars, fats, woody tissue, 
and, in short, almost anything that is found in the bodies of 
animals and plants. The different varieties of microérganisms 
do not all flourish upon the same kind of food. Some seem to be 
able to live upon a large variety of substances, while others de- 
mand particular foods. While almost any kind of proteids will 
serve for the sustenance of the common putrefactive bacteria, 
the tubercle bacillus does not flourish well anywhere outside the 
living body, and, if it is to be cultivated in the laboratory, it 
demands a very special kind of culture medium. But speaking 
in broad terms, the three classes of organisms with which we are 
concerned in our subject, seem to be particularly adapted to dif- 
ferent kinds of food. Bacteria have special relations to proteid 
foods, like lean meat, egg albumen, gluten of wheat, etc., and if 
substances of this nature are consumed by microérganisms, it is 
commonly by bacteria. Yeasts, on the other hand, have a special 
fondness for sugars and, therefore, for starches, which are easily 
changed into sugars. The larger fungi may feed upon either 
proteids or sugars, but they have special relations to the woody tis- 
sues and celluloses of vegetable structures. This classification of 
the foods upon which 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 mush- 
rooms 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 
