36 MORPHOLOGY AND CULTURE OF MICROORGANISMS. 



mixed culture and then draw it repeatedly over the surface of a solid culture medium 

 such as a slice of sterilized potato or a layer of nutrient gelatin in a Petri dish we will get 

 a series of streak cultures. The first of these will develop a strong growth of mixed 

 forms The last will show more and more isolated colonies until some of them will 

 show only a few, some of which may be pure cultures. 



The most useful method of separation and one which is applicable to most cases 

 is that oi'plate cultures, first used by Koch and improved by others. In this method a 

 drop of the mixed culture is thoroughly distributed in 10 to 20 c.c. of liquefied nutrient 

 gelatin or agar. A drop of this mixture is then diluted in the same way in another 

 portion of the same medium. This process is continued until the requisite degree 

 of dilution is obtained. The various portions of nutrient gelatin are then poured, with 

 precautions against outside infection, on glass plates or more conveniently 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 per- 

 centage 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 containing 

 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 con- 

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

 extend over the whole surface (film 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. 



