FOOD OF MICROORGANISMS 155 



intramolecular changes require a special structure. Carbohydrates 

 are the most excellent substances for such intramolecular decomposi- 

 tions. S. cerevisicB and B. coli can live in sugar-free broth only if ex- 

 posed to the air. They provide for all their needs by oxidation of the 

 protein. If oxygen is excluded, growth depends upon sugar, or a 

 similar fermentable compound. We test for the absence of sugar in a 

 given solution by pouring, it in a fermentation tube and inoculating 

 with B. coli: if the liquid in the closed arm remains clear, i.e., if B. coli 

 does not grow without oxygen, it is a good indication that no sugar is 

 present. 



It is usually assumed that in fermentations of this nature, the 

 oxygen atoms are shifted within the same molecule. In other cases, 

 oxygen is taken from one molecule and used for the oxidation of 

 another. This results in one of the molecules being reduced. Nitrates 

 are reduced in this way to nitrites, or amnionia, or nitrogen gas; sul- 

 phates to hydrogen sulphide, and litmus or methylene blue to the 

 colorless leuco-compounds. Such removal of oxygen from a molecule 

 requires energy, and is possible only when the bacterium by using the 

 oxygen for oxidation of organic matter can obtain a larger amount 

 of energy. The following example shows such a possibility: 



2KNO3 + 36.6 Cal. = 2KNO2 -1- O2 

 C2H6OH + 02 = CH3CO2H -}- H2O + IIS Cai. 



This process leaves an energy balance of 115 — 36.6 = 78.4 Cal. for 

 the needs of the bacterium. 



Such decompositions are sometimes referred to as "reducing fermen- 

 tations" but tWs term is not correct, as the reduction must always be 

 accompanied by a simultaneous oxidation process. 



The amount of energy Uberated by a fermentation without oxygen 

 is much smaller than that furnished by complete oxidation; the intra- 

 molecular change always leaves organic compounds which contain a 

 considerable amount of the total energy. Yeast, in presence of very 

 much oxygen, oxidizes sugar completely to water and carbon dioxide. 



CeHizOe + 12O = 6CO2 + 6H2O + 674 Cal. 

 while in the absence of oxygen it will change the sugar to alcohol and 

 carbon dioxide. 



C6H12O6 = 2C2H6OH -1- 2CO2 -f 22 Cal. 



The energy gained in the first process is about thirty times as large 



