78 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



degree of dilution is obtained. The various portions of nutrient gelatin are then 

 poured, with precautions against outside infection, on glass plates or more conven- 

 iently into petri dishes. On cooling and solidifying, the gelatin imprisons every cell, 

 each of which on growing gives rise to a colony. It has been found that in practice 

 a small percentage of these colonies may arise from two adhering cells and thus fail 

 to be pure culture. 



Hansen's modification of the method is intended to obviate this uncertainty. By 

 making the dilutions in the way described for liquid media, a drop of gelatin contain- 

 ing only one cell is obtained, placed on a cover-glass over a culture slide and, by direct 

 observation, the presence of a single cell verified. The development and multiplica- 

 tion of this cell can be watched. 



DIFFERENTIATION OF YEASTS. With magnifications of 300 to 500, yeast cells 

 can be examined conveniently. Contamination with bacteria and molds of special 

 form can be detected, but otherwise a simple microscopic examination is of little 

 value in determining the purity of a culture. Some information regarding the 

 health, nutrition and vitality of the yeast may be obtained and the form of the spores 

 is of some value in distinguishing species. Yeast cells vary in size as much as in 

 form but under standard conditions each variety will show a certain normal range of 

 dimensions. 



If a young, vigorous yeast, in a favorable liquid culture medium, is allowed to 

 remain at rest at a suitable temperature with full access to air and protection from 

 contamination, a growth of cells on the surface will usually take place. This growth 

 may extend over the whole surface (Him formation] or may be restricted to the edges 

 (ring formation) . This growth occurs at once with a few species (S. membrancefaciens) - 

 or at the end of several days (S. ellipsoideus II] or may require several weeks. 

 The time and optimum temperature of film formation have been used as descriptive 

 characters. 



All the morphological and cultural characteristics of yeast are insufficient for 

 diagnostic purposes and must be supplemented by the physiological characteristics 

 such as their action on various sugars and other carbohydrates. 



