CHAPTER VII 



METHODS FOR THE CHARACTERIZATION AND 

 IDENTIFICATION OF YEASTS 



NOW that the methods for isolating the yeasts have been outlined, 

 it is proper to investigate the procedure for determining whether 

 a yeast which one has isolated is a new variety or whether it has 

 been described before, and if it has, in what genus it belongs. We 

 have not an easy task before us. When discussing the morphology of 

 the yeasts, it was pointed out that it was very difficult to distinguish 

 between them. Their shapes are very much the same, varying between 

 a sphere, ellipse and cylinder. With rare exceptions the form and struc- 

 ture of their ascs, and the appearance of their ascospores do not 

 present specific characters. On the other hand the morphological char- 

 acteristics are not constant but subject to variation. The shape and 

 dimensions of the cell vary with the age, the physical and chemical 

 conditions of the environment. It is, then, rather difficult to find 

 in the morphology of the yeast the differential characteristics which 

 permit a close separation of varieties. It becomes necessary to search 

 for distinctive characters among the varieties. We shall have to 

 look to the macroscopic appearance on solid media, to the appearance 

 of the scum on liquid media in contact with air, to the variations pro- 

 duced by the action of various media, and especially, to the biochemi- 

 cal characteristics of the variety. Hansen's investigations have shown 

 that the shape of the cell, the dimensions, and the appearance are, in 

 themselves, sufficiently reliable factors for the identifications of species. 

 To him we owe the solution of this question on specification. To 

 his work, we must look for a great number of characteristics and 

 a method for differentiating between species with all the security desir- 

 able. Hansen has used as determining characters the shape and 

 dimension of the cell at different temperatures and in different media, 

 the shape of the ascospores and their method of germination, the 

 limits of temperature for budding, the formation of a scum, sporula- 

 tion, macroscopic appearances of the scum and of cultures, the 

 biochemical properties and especially their action toward different 

 carbohydrates. Lindner has added to these the very convenient 

 characters determined from the "giant colony." 



We shall now take up in detail the various characteristics which 



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