MECHANISM OF COMPETITION IN YEAST CELLS 63 



nishes the available energy, and in the nutritive medium there takes 

 place a considerable accumulation of the waste product — ethyl alco- 

 hol. If we alter the conditions of cultivation and allow a direct access 

 of oxygen to the growing yeast cells, although fermentation will still 

 continue, a part of the available energy (different for different species) 

 will be furnished by oxidation of sugar into carbon dioxide. In the 

 commercial utilization of yeast, when it is desirable to accumulate 

 alcohol in the culture, yeast is grown nearly without oxygen. But if 

 alcohol is not needed and the object is to obtain a great quantity of 

 yeast cells themselves, an intensive aeration of the growing culture 

 is carried on, which leads to an enormous increase of oxidation proc- 

 esses. The yeast Saccharomyces cerevisiae as well as Schizosaccharo- 

 myces kephir produces alcoholic fermentation, and both can obtain a 

 part of the available energy by oxidation, but they differ from one 

 another in the relative intensities of the oxidation and fermentation 

 processes. Common yeast, Saccharomyces cerevisiae, develops well 

 in the absence of oxygen as for it fermentation is a powerful source of 

 energy. It continues mainly to ferment even in the presence of 

 oxygen (when cultivated in Erlenmeyer flasks without aeration) and 

 utilizes the oxidation process only to a very small extent. As regards 

 our species of Schizosaccharomyces, it grows very slowly under anaer- 

 obic conditions. However, when oxygen is available it has recourse 

 to this source of energy; its rapidity of growth increases and it ap- 

 proaches Saccharomyces in its properties. Hence, Saccharomyces 

 represents a species with distinctly expressed fermentative capacities, 

 whilst Schizosaccharomyces is a species of a more oxidizing type. By 

 mixing these species we obtain a very interesting situation for study- 

 ing the competition between species in different conditions of environ- 

 ment. 



(2) We cultivated yeast in a sterilized nutritive medium which was 

 prepared in the following manner: 20 gr. of dry pressed beer-yeast 

 were mixed with 1 liter of distilled water, boiled for half an hour in a 

 Kochs boiler, and then filtered through infusorial earth. Five per 

 cent of sugar was added to this mixture, and then the medium was 

 sterilized in an autoclave. A medium of such a type is very favorable 

 for the growth of yeast, because the decoction contains all the nutri- 

 tive substances required. The only disadvantage is our ignorance of 

 the exact chemical composition of this medium. Therefore each 

 series of experiments must be made with a solution of the very same 



