CHAPTER XI 

 BIOLOGICAL ACTIVITIES OF YEASTS 



ECOLOGY OF YEASTS 



We find yeasts in nature wherever sugar is present, in the various 

 foodstuffs of man, in the nectar of flowers, the exuded sap of trees, 

 and above all on the surfaces of fruits. They are also found in soil, 

 on the leaves of plants, and as symbionts or parasites in various 

 animals, especially insects. 



The constant occurrence of yeasts on fruits, especially grapes, 

 naturally led to inquiries as to their origin. Pasteur showed that 

 the immature fruits are free from yeasts, and that, if they are cov- 

 ered with cotton while ripening, no yeast will develop. Hansen be- 

 lieved, as a result of investigations with Hansenia apiculata, that the 

 yeasts remain dormant in the soil during the winter and spring, prob- 

 ably in the form of ascospores, and are deposited on the fruit by the 

 wind. The spores germinate and bud there to form new spores on 

 the overripe fruit at the approach of winter. He found yeasts much 

 more numerous in the soil of vineyards than elsewhere. However, 

 Starkey and Henrici *^ found yeasts to be relatively infrequent in 

 soil, being present in very small numbers in only 38 of 87 soil 

 samples of all kinds, including a number from orchards. The vari- 

 eties found were for the most part not those kinds found to be active 

 in the spontaneous fermentation of fruit juices, only four cultures 

 of Saccharomyces being found. 



Yeasts have been found very frequently on insects and in their 

 digestive tracts, and it is probable that insects are more important 

 in distributing yeasts on fruits than is the chance distribution of in- 

 frequent yeasts from soil by the w^nd. Since yeasts are apparently 

 distributed in nature largely through the agency of insects, it is not 

 surprising to find yeasts frequently present in their alimentary tracts. 

 Thus Berlse believed that the intestinal tract of Diptera was the 

 normal habitat of many yeasts, including "Saccharomyces ellip- 

 soideus." It has also been shown that the yeasts form an important 

 part of the diet of certain insects, such forms as the fruit flies (Droso- 

 phila) for instance, depending for their nutrition not so much upon 



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