EXTRACTS OF PURE DRY YEAST 97 



ent in the yeast cell, which makes them valuable for general bac- 

 teriological purposes. The fermentable material probably stim- 

 ulates growth but renders the extract valueless as an ingredient 

 of media for fermentation tests. 



For use in a medium for fermentation tests, the dry fresh yeast, 

 at least the preparation used in our work, may be autolized in the 

 laboratory. In this process all but a trace of the fermentable 

 material is destroyed and the small amount left does not inter- 

 fere in any way with the determination of the fermentation of 

 test substances even in a lightly buffered medium. 



Yeast extracts can be made by treatment with acid or alkali or 

 by peptic or tryptic digestion. The value of extracts made by 

 digestion with HCL and pepsin are only indicated by the experi- 

 ments reported but the results are promising. 



Dry fresh yeast can be readily obtained and when prepared 

 under definite conditions should be uniform in composition. Its 

 keeping quality and ease with which it can be procured appar- 

 ently make it much more valuable than ordinary undried brew- 

 er's yeast. Whether extracts from fresh brewer's yeast possess 

 any advantages over extracts from dry yeast as a culture medium 

 remains to be determined. 



Dry autolized yeast has been prepared for us by a yeast manu- 

 facturer which in combination with peptone proved to be an ex- 

 cellent medium for the growth of streptococci, particularly patho- 

 genic types, which would not grow in other sugar free media. 

 This medium was practically free of fermentable material and 

 for this reason was valuable for fermentation tests. 



It is not the purpose of this paper to convey the idea that the 

 yeast extracts can be used to replace all other ingredients in 

 media. While they can be used to advantage both alone and as a 

 basis for more complicated media, it has been found that they 

 are more useful with some organisms than with others. How- 

 ever fragmentary our knowledge of the value of yeast extracts 

 for culture media, the results of other investigators aDd our 

 own experience clearly point to the desirability of giving much 

 more attention to what appears to be a very valuable subject. 



THE JOURNAL OF BACTERIOLOGY, VOL. V, NO. 1 



