156 NUTRITION AND METABOLISM 



as that gained in the second' process. This was demonstrated as early 

 as 1 86 1 by Pasteur. He grew yeast in sugar solutions, varying only 

 the amount of oxygen in contact with the medium. At the end of 

 the experiment, the weight of the dry yeast and the > decomposed sugar 

 was determined, and the amount of sugar necessary to produce one 

 part of yeast was computed. He found: 



In a closed flask, without any air i part yeast required 176 parts sugar. 



In a closed flask, with large air space i part yeast required 23 parts sugar. 



In a thin layer, a few mm. thick .1 part yeast required 8 parts sugar. 



In a very thin layer, in 24 hours i part yeast required 4 parts sugar. 



- This "experience led Pasteur to the conclusion that fermentation 

 corresponded to the respiration process of animals, that fermentation 

 was respiration without oxygen. 



It is quite evident that since the utilization of the food in the 

 absence of oxygen is very high, the organisms have to decompose 

 -much more food. This accounts, to a great extent, for the enormous 

 destructive power of bacteria, when comparisons of the great quantity 

 of food decomposed are made with the very insignificant weights of 

 cells. It has been estimated that the lactic bacteria decompose their 

 own weight of sugar in one hour. 



Summing up the relation of oxygen to microorganisms, some 

 bacteria, and especially the molds, are found depending upon oxygen as 

 an indispensable part of their food. Three groups are recognized: 

 Those, a large number, organisms in the presence of oxygen producing 

 oxidations; those able to sustain life without oxygen; and those de- 

 pending entirely upon decompositions which require no oxygen. 

 The lactic bacteria and the butyric bacteria belong in the last group. 



In considering the oxygen requirements, it is customary to in- 

 clude another influence of oxygen upon bacteria. This has really 

 nothing to do with its food value, but deals with the poisonous qualities 

 of oxygen. Oxygen in this light may well be called a poison as it will 

 kill bacteria in very low concentrations. Ordinarily it is regarded as 

 constituting over 20 per cent of our atmosphere. But if a study is 

 made .of its effect upon bacteria, it is necessary to measure it in the 

 same way food 'is measured, and consider the concentration in whish 

 it is offered to the cell. Microorganisms obtain their oxygen not as 

 gas, but as dissolved oxygen. The solubility of oxygen is very small, 

 about 0.0009 P^'" cent at 20°. Practically aU bacteria die readily if the 



