332 PUBLIC HEALTH BACTERIOLOGY 



formed by endogenous cell-division, and the usual rule 

 is for the protoplasm of one cell to divide into four spores, 

 each with its own cell-membrane, the original cell- 

 membrane persisting and serving as an envelope enclosing 

 the spores. 



Yeasts grow more slowly than bacteria, and are hence 

 more difficult to isolate from mixed cultures on the 

 ordinary media. Once isolated they are kept alive by 

 subculture every 2 to 3 months. On glucose agar or plain 

 agar, colonies appear in 3 to 4 days as minute glistening 

 white spots. In stab, the growth is all at the top, forming 

 a heaped-up creamy layer on the surface of the medium. 

 In broth, a stringy gelatinous growth is formed. Growth 

 takes place on gelatin, which is not liquefied. On potato, 

 growth is more rapid. The cultivated yeasts used in 

 brewing and baking processes are capable of fermenting 

 various sugars. This action is due to certain ferments or 

 enzymes elaborated by the yeasts. Two of the enzymes 

 are diffused into the medium in which growth is taking 

 place ; one remains closely bound to the yeast cell, and 

 was only isolated by Buchner by rupturing the living 

 yeast cells under great pressure, filtering, and centrifugal- 

 izing the filtrate. This filtrate was found to have the 

 power of fermenting glucose and laevulose into alcohol 

 and carbonic acid gas. This endo-enzyme is called 

 " zymase." 



C 6 H 12 O 6 = 2-C 2 H,-OH +>CO 2 



The other two soluble enzymes are " invertase " and 

 " maltase." The former inverts cane sugar or saccharose 

 into invert sugar (glucose + laevulose), and so renders it 

 susceptible to the action of the " zymase." " Maltase " 

 acts on malt sugar, changing it into glucose, which is then 

 acted on by the " zymase." 



Q-2H 22 O U + H/) = C 6 H la O 6 + CgH^Ofi 

 Saccharose. Glucose + laevulose. 



C 12 H M O tl + H a O = C fi H ia 6 + QH 12 6 

 Maltose. Glucose -f glucose. 



Saccharomyces cerevisiae. This is the name applied 

 to the yeast in common use by brewers and bakers. 



