SACCHAROMYCETACEAE 



305 



Determination of Species. — Two media have been widely used iu studies 

 of yeasts : malt extract and yeast decoction. The former is prepared as fol- 

 lows (Guilliermond 1928) : 



Barley is germinated on moist blotting paper on plates. When the grain 

 has swollen and the radicle has begun to emerge, it is dried in an oven at 

 30° C, and ground in a mortar. Two hundred grams of powder are stirred 

 into a liter of cold water which is slowly heated to 60°, and this temperature 

 is maintained for 45 minutes, with occasional stirring of the mixture. Four 

 grams of hops are added and the whole boiled for about an hour. The result- 

 ing decoction is then sampled, the amount of maltose determined, and the 

 whole diluted to bring the concentration of maltose to 3%. It is then sterilized 

 in the autoclave at 120° C. The decoction is usually used to prepare either 

 1.5% agar or 6-15% gelatin. The malt extracts now on the market seem 

 suitable and have yielded good results in our laboratory. The preparation 

 of media from them is much less tedious. 



Yeast decoction is prepared by boiling and stirring 100 gm. fresh yeast 

 in 1 liter of distilled water, filtering, and autoclaving. The liquid furnishes 

 a small amount of available nutrient so that some sugar is often added. 



j?ig. 63. — Saccharomy codes Ludioigii. Copulation and development of asci (X750). (After 



Guilliermond 1905.) 



The macroscopic characters on malt extract incubated for 24 hours at 

 25° C. are studied. Some yeasts form a white sediment in the bottom of the 

 test tube, while others grow mostly at the surface, forming a pellicle or, if 

 they creep up the sides of the tube, a ring. If the tube is not handled care- 

 fully, the pellicle may break up and settle as a sediment, but if left alone a 

 new pellicle will soon form. Often the pellicle is characteristically folded, 

 etc. Fermentation should also be observed if present. 



The microscopic characters are more variable. One should observe the 

 general shape and size of the cells, the method of budding, whether only 

 terminal or lateral, whether a pigment is characteristically produced. For 

 variation in appearance of cells of different ages, the following account is 

 summarized from Shrewsbury (1930) : 



The adolescent cell (Fig. 40, 1-3) is spherical to allantoid in shape with 

 a thin cell wall and refractile, homogeneous cytoplasm. Sprouting is usually 

 most active at the poles, but may occur anywhere. The mote cell is larger 

 than the adolescent cell, and the cytoplasm more granular. A single vacuole, 



