94 S. HENRY AYERS AND PHILIP RUPP 



Doubtless, the increase in amino acids and other nitrogenous 

 material makes autolized yeast a more valuable medium than 

 dried fresh yeast for many purposes. This is, of course, true 

 when fermentations are being studied. However, in combina- 

 tion with peptone when abundant growth alone is desired, the 

 extract of dried fresh yeast and peptone medium showed more 

 vigorous growth with all organisms tested. 



As far as our experiments are concerned, the value of an auto- 

 lized yeast peptone broth lies in the fact that it gives a sugar 

 free medium in which streptococci, particularly the pathogenic 

 types, will grow readily. As most of our pathogenic streptococci 

 would not grow in the ordinary extract peptone broth, the yeast 

 medium proved to be of great value in fermentation tests. 



Since dried autolized yeast could not be obtained commercially, 

 experiments were conducted to determine if the dry fresh yeast 

 could be autolized in the laboratory. Ten grams of dry fresh 

 yeast were mixed with 200 cc. of distilled water and incubated 

 at 42°C. for twenty-three hours. The dry yeast used had not 

 been heated to 105°C. but was in the condition received from the 

 manufacturer. After incubation this yeast was heated to 100°C. 

 for one-half hour then cooled and filtered. The filtrate which 

 was turbid was then cleared by the addition of kieselguhr and a 

 second filtration. The extract was then made up to the proper 

 amount to represent 1 per cent of dried yeast. One per cent 

 peptone was added and the reaction corrected to pH 7.5 as in 

 the media previously described. Analysis showed that the ex- 

 tract from the autolized yeast was practically identical with that 

 made from the dried autolized yeast mentioned above and it 

 acted the same in cultural tests. 



It is evident that the dried yeast preparation used in this work 

 can be readily autolized in the laboratory and can be so handled 

 as to give uniform results. 



Yeast manufacturers are urged to give attention to the prep- 

 aration of autolized yeast for use in bacteriological work. Dernby 

 (1918) has been able to show that in yeast cells there must be at 

 least three different groups of proteolytic enzymes acting at dif- 

 ferent hydrogen ion concentrations: pepsin, splitting proteins 



