226 NUTRITION AND METABOLISM 



In some of the more complex fermentations, we find simultaneous 

 formation of hydrogen or methane and carbon dioxide; the one is 

 the end product of reduction, the other the product of complete oxida- 

 tion. This also indicates that the oxidation of one part of the molecule 

 takes place at the expense of the other. 



In a similar way, some organic acids, e.g., tartaric and lactic acids, 

 can be fermented by certain bacteria without requiring oxygen. Some 

 bacteria have the ability to attack proteins and decompose them 

 completely in the absence of oxygen. 



Bacteria, having the ability to provide for their energy without 

 oxygen gas, may live in the complete absence of oxygen, and may 

 multiply indefinitely without it as long as there is sufficient food. But 

 some microorganisms, such as yeasts, seem to grow only for a limited 

 time in the absence of oxygen. Finally, they cease growing, and 

 we may well assume that they need oxygen for cell construction which 

 can be used in no other form except as molecular oxygen. The urea 

 bacteria also belong in this group. 



A large number of bacteria and yeasts, and also a few molds, can 

 provide for their energy by either oxidation or decomposition in the 

 absence of oxygen. Very commonly a great variety of compounds can 

 be found which may be oxidized while but very few can be intra- 

 molecularly fermented without oxygen. This is easily understood: 

 all organic compounds will yield heat upon oxidation, while exothermic 

 intramolecular changes require a special structure. Carbohydrates 

 are the most excellent substances for such intramolecular decomposi- 

 tions. S. cerevisice and B. coll 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. coll: if the liquid in the closed arm remains clear, i.e., if B. coll 

 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 ammonia, or nitrogen gas; sul- 



