DETERMINATION OF BACTERIA IN MILK 567 



counted and the total count obtained as well. It was found 

 that the total counts on the casein medium were decidedly lower 

 than on infusion agar unless the plates were incubated for six 

 days. This fact, of course, made the medium almost valueless 

 for routine control work. Attempts to make a medium by using 

 skimmed milk, with its casein dissolved, and adding peptone and 

 extract were not successful. 



Having in mind this casein medium, with its advantages and 

 defects, it seemed probable that skimmed-milk powder could 

 be used in place of purified casein, and that with some additional 

 nutritive substances a cheap and valuable medium could be 

 prepared which, in forty-eight hours, would give counts at least 

 as good as the present standard extract agar and also give a 

 count of the peptonizers present. 



In this work about 50 different media were made, containing 

 milk powder alone and in combination with meat extract, yeast 

 extract, peptone, peptone and meat extract, and meat infusion. 

 Ordinary agar was tried, also agar with some of the calcium and 

 magnesium salts removed by precipitation with sodium dibasic 

 phosphate, and agar in which they were partly removed by 

 washing. 



The media suggested in this paper are not the result of a "lucky 

 shot." They were developed during a study of various com- 

 binations which were in every case compared with the standard 

 extract agar. This work resulted in the examination of about 

 400 samples of milk of different kinds, that is, raw, pasteurized, 

 and certified; which, figuring the different combinations, is equiv- 

 alent to the plating of about 1750 samples of milk on a single 

 medium. 



This is mentioned only to impress upon the reader that the 

 directions given for making the media are based on considerable 

 experimental data which it is impracticable to include in this 

 paper. We do not feel that the media are beyond improvement, 

 but hope the directions will be followed until they have been 

 given a fair trial. 



We will describe a skimmed-milk powder medium containing 

 meat extract and peptone, a modification of this medium, and 

 a milk powder medium containing no meat extract or peptone. 



