90 S. HENRY AYERS AND PHILIP RTJPP 



which was sugar free and supported the growth of bacteria, 

 among which were pathogenic streptococci, made it very useful 

 until the preparation was found in the year of 1918 to contain 

 starch which was evidently added during the process of manu- 

 facture. The usefulness of extracts of this preparation for fer- 

 mentation tests was, therefore, ended although for general bac- 

 teriological purposes it was still valuable. 



A dry yeast was then obtained through the kindness of a man- 

 ufacturer 1 in this city which has given valuable and interesting 

 results. The dry yeast consists of pure washed fresh yeast and 

 was found to contain about 6.0 per cent of moisture. 



In our experiments, the value of yeast extracts for replacing 

 meat extract have been given most attention, therefore they 

 have been used largely in combination with peptone. Various 

 types of streptococci were used to test the value of the yeast 

 medium, since they are among the most difficult of the bacteria 

 to grow in culture media. Many other organisms were found to 

 grow more readily than the streptococci. 



The yeast extract was prepared by mixing 1 per cent of dry 

 fresh yeast with cold distilled water and after being allowed to 

 stand 10 minutes was steamed in an Arnold sterilizer for thirty 

 minutes, and then filtered. Considerable difficulty was encoun- 

 tered in obtaining a clear filtrate, the liquid always appearing 

 hazy. The haziness could be removed by the addition of kiesel- 

 guhr and a second filtration. Probably Merck's dialyzed iron 

 could be used to advantage for clearing the extract as has been 

 mentioned by Eberson (1919), who pointed out the value of a 

 yeast medium for prolonging the life of the meningococcus. 



It was found that the difficulty in obtaining a clear extract could 

 be overcome by heating the dried yeast before use at 105°C. for 

 four or five hours. After the first steaming, the extract appeared 

 cloudy upon filtration but when the reaction was adjusted to 

 about pH 7.7 and the extract steamed a second time, a precipitate 

 was produced which could be readily filtered out leaving a clear 

 solution. 



1 This opportunity is taken to express our thanks to R. L. Corby and Miss 

 Glasgow of the Corby Company, for supplying us with preparations of dry yeast. 



