INTR0DX7CTI0N 147 



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, and they cannot 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 favor a good develop- 

 ment of the bacteria. The malic acid is used entirely for the forma- 

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

 urea fermentation. This example shows clearly the different require- 

 ments for cell growth and for the energy sUpply. 



With the urea fermentation, we have changed not only from inor- 

 ganic to organic food, but also from oxidation processes to other 

 decompositions. 



Microorganisms differ from the higher animals by their less complete 

 metaboUsm. The food in the animal, if digested at all, is oxidized as a 

 rule to the final products of combustion, CO2 and H2O, the only excep- 

 tion being the nitrogen which leaves the body still in organic combina- 

 tion as urea. With bacteria, yeasts and molds, this is not always the 

 case. Though some of these organisms will bring about complete oxida- 

 tion of the food we find more commonly incomplete oxidations or 

 changes which require no oxygen at all, but still 3deld energy to the cell. 

 The biochemical side of these changes of which the alcoholic fermenta- 

 tion is the best known will be discussed in the chapter on oxygen 

 requirements. 



