IDENTIFICATION OF YEASTS 219 
Physiological Method. In this method an environment is created 
which is favorable to the desired fungus and unfavorable to the others. 
In applying this method to the yeasts, an acid medium may be used. 
Bacteria, unless they are acidophiles, are unable to grow in a 2 per cent 
tartaric acid solution. Pasteur and Hansen grew yeasts in such a 
medium to free them from bacteria. This method was accompanied 
with some uncertainty and was replaced by the dilution methods. 
Dilution Methods. These have been described before. They con- 
sist of diluting or breaking up the yeast’s masses in a definite amount of 
sterile water or physiological salt solution. From these diluted mix- 
tures definite portions are taken for plating on a solid medium. After 
incubation and growth, cultures should be picked from the colonies and 
transferred to dextrose agar slants. The yeast is then ready for a cul- 
tural and morphological examination. 
Moist Chamber Preparations. These are essentially the hanging 
drop mounts which have been described before. A small portion of 
the pure culture of yeast should be gently emulsified in a drop of dex- 
trose broth or wort. This should be transferred to a clean cover glass 
and suspended over a depression in a concave slide or ring slide. The 
edges of the cover glass should be carefully lined with vaseline in order to 
make a tight seal and allow no evaporation during incubation. Sucha 
mount will permit observations on a few cells to allow determine bud- 
ding and cell shape. 
Scheme for Identifying a Yeast. Whether the fungus is really a 
yeast or borderline mold should be determined first of all. The following 
points may be of assistance: 
I. Spore or non-spore former 
A. Spore former 
1. Shape, morphology 
2. Scum or membrane 
3. Top or bottom yeast 
4. Per cent of alcohol formed 
B. Non-spore former 
1. Non-scum former 
a. Color 
b. Shape, morphology 
2. Scum former 
a. Color 
b. Morphology 
