20 THE GENERAL CHARACTERS OF MICROORGANISMS. 
along upon a smaller amount of water. Substances that are too 
dry to putrefy may be spoiled by molding; hence it is much more 
difficult to preserve certain kinds of partly dry food from moulding 
than from decaying. Flour, in a flour barrel, may become musty 
from the development of molds, but it will hardly show signs of 
decay or putrefaction unless it becomes actually wet. 
Relation to Food. Bacteria and the other fungi feed upon an 
immense variety of foods. A few of them are able to nourish 
themselves upon mineral matter, and some can gain their necessary 
carbon from the small amounts of carbon dioxide in the air, re- 
sembling, in this respect, the green plants that are engaged in build- 
ing up starch and other organic compounds. This class of bacteria 
is of great theoretical interest and doubtless of much importance in 
nature. But the vast majority of microorganisms are unable to use 
mineral foods, requiring, like animals, to 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 microorganisms do not all flourish upon the same kind of 
food. Some seem to be able to live upon a large variety of sub- 
stances, while others demand particular foods. While almost any 
kind of proteids will serve for the sustenance of the common putre- 
factive 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 different 
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 microorganisms, 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 tissues and celluloses of 
vegetable structures. This classification of the foods upon which 
