2O 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 



