LIFE CYCLES 1-7 



the collection of milk by humans. As far as I know, there are no 

 common yeasts existing in the udder, and yeasts are not generally 

 associated \^ith milk in the body. S. lactis beta does not ferment 

 lactose but digests milk proteins; it may have a similar recent ori- 

 gin but depends principally on the protein rather than the carbohy- 

 drate component of the milk for its nutrition. 



Saccharomyces cerevisiae and S. carlsbergensis may be very 

 ancient. They are adapted to grow in the natural sugary solutions 

 which contain enough acid to inhibit most bacteria and enable the 

 yeasts to get started before other microorganisms. They are found 

 on dates or figs, apricots, grapes, apples, and pears, or any other 

 kind of spoiled fruit. 



Saccharomyces, Schizosaccharomyces and Saccharomycodes are 

 "perfect" yeasts. They are able to produce ascospores and, there- 

 fore, have a complete life cycle, haploid to diploid. 



There are many "imperfect" yeasts, so-called because they do 

 not have a complete sexual cycle. Torulopsis, Cryptococcus, and 

 Torula (fig. 1-5) are synonyms for a genus which produces vege- 

 tative cells that never sporulate. Torulospora, a very similar ge- 

 nus, sporulates but is otherwise close to Torulopsis. These strains 

 are generally distinguishable from the perfect Saccharomyces. The 

 specific names are convenient means of classifying these organisms, 

 but the classification of imperfect fungi has limited phylogenetical 

 significance. 



Torulopsis is adapted to grow best in a well -aerated medium. 

 Most species of this genus ferment only a few sugars; some even 

 fail to ferment glucose. The famous Torulopsis utilis was discov- 

 ered by Henneberg (1926) as a contaminant of baking yeast plants 

 in Germany. Henneberg realized this new yeast provided a very 

 rapid means of synthesizing proteins from ammonia and sugar. 



Another organism derived from the haplophase of Saccharomy- 

 ces is Zygosaccharomyces (fig. 1-6). It produces cells resembling 

 rather closely the haplophase of Saccharomyces. Under proper con- 

 ditions the vegetative cells of Zygosaccharomyces (which are ob- 

 viously haploid) copulate with each other to produce the diplophase; 

 zygotes and spores are produced immediately in the dumbbell shaped 

 ascus. Some species are adapted to grow in very hypertonic sugar 

 solutions. They are important industrially as contaminants in honey, 

 which may contain as much as 65 per cent sugar. Winge and Laust- 

 sen (1939) produced hybrids between Zygosaccharomyces and Sac- 

 charomyces by spore to spore matings; this significant fact shows 

 the close relationship between these genera. 



There are a large number of minor Saccharomyces which can 

 be used as sources cf genetical characteristics: Saccharomyces 

 bayanus is an interesting species; instead of elliptical cells, like 

 S. cerevisiae, it has cylindrical cells. This character segregates 



