11-7 THE YEAST CELL 



guished fermenters and synthesizers from cultures conpletely 

 incapable of fermentation or synthesis, as will appear in the sub- 

 sequent discussion. 



FERMENTATIVE ABILITY 



The ability of Saccharomyces to ferment different sugars is 

 gene -controlled and our breeding stocks are differentiated by their 

 abilities to ferment galactose, melibiose, maltose, sucrose, raf- 

 finose and methyl alpha glucoside. We have used these fermenta- 

 tive differences as genetical markers and have concluded that a 

 single gene -pair differentiates two stocks when in a large number 

 of tetrads isolated from a hybrid, two cultures from each tetrad 

 are "fermenters" and two are "nonfermenters." 



THE FERMENTATION TEST 



The fermentation test is performed in a 12 x 100 mm. Durham 

 fermentation tube containing 3 cc. of a 4 per cent concentration of 

 the carbohydrate and an inverted insert, 7 x 50 mm. The Durham 

 tubes are prepared as follows: Each receives 2 cc. of the following 

 medium without added sugar, plugged with cotton and sterilized 

 with the inverted insert in place: 



One cc. of a Seitz -filtered 12 per cent solution of Pfanstiehl 

 carbohydrate is added to make a final concentration of 4 per cent. 

 The Durham tube is inoculated with a large loop of organisms 

 from an agar slant (fig. 11-2). Most yeasts are able to ferment 

 dextrose, fructose, and mannose, and any hydrolysis whi.;h splits 

 off one of these hexoses, results in a positive fermentation 

 test. 



