584 



S. HENRY AYERS AND COURTLAND S. MUDGE 



TABLE— 3 Continued 



EXTRACT AGAR, 



STANDARD METHODS 



A. P. H. A. 1916 



INFUSION AGAR, 



STANDARD METHODS 



A. P. H. A. 1910 



INCREASE 



OVER EXTRACT 



AGAR 



MILK-POWDER TEA8T 

 AGAR 



INCREASE 



OVER EXTRACT 



AGAR 



A medium with skimmed-milk powder and washed agar with- 

 out any other nutritive material gives counts much higher than 

 the standard extract agar. As may be seen in table 4, the counts 

 were much higher except in the case of the last sample. In 

 many samples the counts on the milk-powder agar, with no 

 other nutritive ingredients, checked with those on standard 

 infusion agar, while with a few samples the count was decidedly 

 lower. The colonies, however, on this medium are very small 

 in most cases; therefore, it is necessary to supply some other 

 nitrogenous material to stimulate growth. 



Peptone and meat or yeast extract, whichever may be used, 

 are added in order to increase the nutritive value of the medium. 

 Milk powder supplies lactose to the medium in small amounts, 

 which assists materially in stimulating and supporting bacterial 

 growth, as has been shown by the work of Sherman (1916) and 

 others. We do not feel, however, that the lactose is the only 

 reason for the high counts obtained on media containing dis- 

 solved milk powder. 



The increased count over extract agar and the large size of 

 the colonies obtained on the media described in this paper are 



