90 NUTRITION AND METABOLISM. 



Quite large in comparison, though, is the amount of food used to provide 

 for the energy requirements of the cell. The quantities of protein matter 

 decomposed in soft cheese, of sugar destroyed by alcoholic and lactic 

 fermentations, are very large and easily determined analytically. 



NON-NITROGENOUS FOOD COMPOUNDS. The simplest carbon com- 

 pound, carbon dioxide (CO 2 ), cannot possibly be used as a source of 

 energy, because it cannot be decomposed with liberation of energy. It is 

 used for cell construction by a few bacteria, e.g., the nitrifying and the 

 sulphur bacteria. This must be considered an unusual occurrence, 

 however, since nearly all other bacteria and all yeasts, molds and protozoa 

 depend on organic matter for their cell construction. Methane (CH 4 ) 

 can be used by one or two bacteria for growth as well as for energy, and 

 even hydrogen gas serves as food, together with carbon dioxide, to one 

 bacterium. As a general rule, however, hydrocarbons are not attacked 

 by microorganisms. The compounds containing oxygen in addition to 

 carbon and hydrogen are better adapted for microbial food. The simple 

 alcohols can be used only by a few microorganisms, while the more com- 

 plex alcohols, like glycerin (C 3 H 5 (OH) 3 ), mannit (C 6 H 8 (OH) 6 ), etc., 

 are very valuable as food for most molds, yeasts and bacteria. Apparently 

 the best nitrogen-free food compounds for microorganisms are the 

 carbohydrates, especially the hexoses, C 6 H 12 O 6 , and the bioses, C 12 H 22 O U . 

 They can be decomposed in many different ways, always yielding energy. 

 They are also very valuable as material for cell construction, and as 

 mentioned in the introduction to this chapter, the bioses are better adapted 

 for cell construction while the monoses are more easily fermented. The 

 insoluble carbohydrates like starch and cellulose are not as generally 

 decomposed by microorganisms as the soluble carbohydrates, though 

 many species have the ability to attack them. 



Organic acids are excellent food for the microorganisms having strong 

 oxidizing properties, since oxidation is almost the only process of decom- 

 position that will yield energy from acids. Some of the dibasic organic 

 acids [succinic acid (CO 2 H' CH 2 'CH 2 -CO 2 H) and tartaric acid (CO 2 H'- 

 CH(OH)-CH(OH)'CO 2 H)] are quite commonly used as building mate- 

 rial ; they are often added to culture media for microbial plants. Ordinar- 

 ily, fats are not easily attacked. They are first split into their components, 

 glycerin and fatty acids, and the glycerin is decomposed readily while the 

 acids are used up very slowly. Other organic compounds may be used 

 occasionally by certain microorganisms, and probably there is no organic 



