INTRODUCTION. 85 



Besides, some more complex compounds, like sodium tetrathionate 

 (Na 2 S 4 O 6 ), are formed. The bacterium builds its cells exclusively from 

 nitrates, carbon dioxide, and mineral salts; organic food is rejected. The 

 hyposulphite can hardly be used for the construction of the cell, and must 

 be considered entirely a food for energy. 



This distinction is not confined to mineral decomposition only. The 

 urea bacteria get their energy from the decomposition of urea into 

 ammonium carbonate. 



(NH 2 ) 2 CO + 2H 2 O - (NH f ) 2 CO 3 



Urea Ammonium carbonate 



But the urea and mineral salts are not sufficient for the development of 

 the urea bacteria. They cannot use urea as a material for building the 

 cells, nor can they use carbon dioxide or carbonates; they cannot grow 

 unless a suitable material for cell construction is added. Sohngen 

 demonstrated that a few milligrams of malic acid allow a good develop- 

 ment of the bacteria. The malic acid was used entirely for the formation 

 of cell substances. The energy for this formation came from the urea 

 fermentation. This example shows clearly the different requirements 

 for cell growth and for the energy supply. 



This difference exists also in the alcoholic fermentation by yeasts. 

 Recently Lindner and Saito have shown that some yeasts ferment certain 

 sugars, and thus can obtain energy, but they cannot use them for growth. 

 They depend upon other organic substances for cell building material. 

 Other sugars can be used for growth, but cannot be fermented. As a 

 general rule, the monoses of the formula C 6 H 12 O 6 are most easily fer- 

 mented, while the bioses with the formula C 12 H 22 O n , especially maltose, 

 are much better adapted for growth. 



Generally speaking, microorganisms obtain their energy by causing 

 chemical decompositions which may be either oxidations (nitrification, 

 vinegar fermentation) or intramolecular changes (urea fermentation, 

 alcoholic fermentation). Many organisms are able to cause various 

 decompositions. Yeasts will ferment sugar to alcohol and carbon dioxide 

 if air is completely or largely excluded, but if the yeast culture is well 

 aerated, the larger part of the sugar is oxidized completely to carbon 

 dioxide and water, and the alcoholic fermentation is retarded. This 

 double action upon sugar is still more pronounced with many representa- 

 tives of the mucor family. They oxidize in presence of air and cause 



