370 Microbes and You 



Yeasts are non-motile and are facultative with respect to their 

 oxygen requirements. Because of their relatively large size (range 

 of 3 to 10 /x wide and 3 to 100 fx long), more of the structures of 

 yeasts can be seen under the oil immersion objective than is true 

 of most bacteria. In spite of their unicellular make-up, yeasts are 

 considered to be a little higher in the scale of plant life than bac- 

 teria—perhaps between the bacteria and the molds. In general, 

 the technics employed for the cultivation of bacteria are the same 

 for yeasts. 



It was Leeuwenhoek in 1680 who first described yeasts as living 

 things, but it was not until the fundamental work of Pasteur in 

 1865 that the role of these funo^i in fermentation was established. 



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In 1881 Hansen explained how yeast cells, found as part of the 

 normal flora on ripening grapes, spent the winter in the soil, prob- 

 ably in the spore stage. He felt that these spores were blown 

 about on dust particles with the aid of wind currents, and the 

 following growing season they were transported to the grapes by 

 this same means. Qualitative bacteriological studies conducted 

 on the digestive tract of insects will frequently disclose the pres- 

 ence of yeasts, and with insects serving as incubators during the 

 cold winter months, fungi may survive and eventually be de- 

 posited on grapes and other fruits by the activity of these insects. 



CLASSIFICATION OF YEASTS 



It is to be hoped that the changes and the counter-changes in 

 the naming and the classifying of yeasts will eventually settle down 

 and become standardized and accepted by microbiologists. A 

 single species masquerading under several different names is verv 

 confusing, particularly to those persons who are approaching the 

 study of yeasts for the first time. 



For the sake of simplicity yeasts may be considered as existing 

 in just two major groups: the family Endomijcetaceae, which in- 

 cludes those cells which produce ascospores and are known as 

 TRUE YEASTS, and the family Tondaceae which do not fonn asco- 

 spores and are known as pseudo (false) yeasts. Subgroups in 

 these families are based principally upon morphology— the size and 



