62 AGRICULTURAL BACTERIOLOGY 



energy contained in its food. In the case of the organism 

 that forms by-products that contain much energy, the 

 weight of the cells will be insignificant in comparison to 

 that of the by-products. The first type of organism will be 

 of no value in the manufacture of products of industrial 

 value, nor will it be of importance in disease production, 

 except as it may cause mechanical injury. It is this prop- 

 erty of partial decomposition of their food that gives to 

 the bacteria and the yeasts their great importance in indus- 

 trial fermentations, and that makes the bacteria such potent 

 agents in the causation of disease. The animals and the 

 strictly aerobic forms of microorganisms carry on the de- 

 composition of their food to a very complete degree. In 

 other words, their waste products are simple in composition, 

 and contain little energy. It should be kept in mind that 

 the microorganisms derive both building material and 

 energy from their food. Certain constituents of the food 

 may contain no energy, and can, of course, be used only 

 for the formation of the cell substance. Other foods may 

 serve both purposes. Indeed, the greater part of the food 

 will be thus used. It is customary to speak of food for 

 energy and of food for growth. 



Classes of organic matter. The greater part of vegetable 

 and animal matter is to be included under the following 

 great groups : carbohydrates, which .include the celluloses, 

 the starches, and the sugars, compounds containing carbon, 

 hydrogen, and oxygen; proteins, which in addition to the 

 above elements also contain nitrogen, sulphur, and some- 

 times phosphorus; and the fats, which contain the same 

 elements as the carbohydrates. The latter are, however, less 

 easily decomposed by microorganisms than are the carbo- 

 hydrates. The chemical transformations that these organic 

 compounds undergo as they are acted upon by microorgan- 

 isms can be most clearly represented in connection with the 



