DETERMINATION OF BACTERIA IN MILK 587 



the same as standard extract agar, with the exception of medium 

 B, which costs only about 6 cents a liter. It seems evident that 

 no objections can be raised to milk-powder media on the ground 

 of cost. 



SUMMARY 



1. Formulae for three agar media are given, two of these media 

 containing skimmed-milk" powder with different amounts of 

 peptone and meat extract. The third contains skimmed-milk 

 powder and yeast extract, no peptone or meat extract being 

 used. This may be a distinct advantage if the variation in 

 composition of peptone and meat extract plays an important 

 part in connection with the bacteria count. 



2. Any of the skimmed-milk powder media described give 

 counts very much higher than standard extract agar and the 

 colonies are very much larger. The larger size of the colonies 

 makes the counting process much more accurate. Counts 

 obtained on the powder media w T ith peptone and extract are 

 on the average higher than those obtained on the old standard 

 infusion. On milk-powder yeast agar they may be, in general, 

 slightly lower. 



3. With these milk-powder media it is possible to obtain not 

 only a total count, but a count of colonies of strong and weak 

 acid-producing, alkali-forming, inert and peptonizing bacteria. 

 The plates therefore, give quantitative and qualitative results, 

 at least so far as these groups of bacteria are concerned. 



4. The importance of using washed agar in media for bacterial 

 counts will be discussed further in another paper. 



5. It is hoped that these media will be given a thorough trial, 

 for it is felt that any one of the three possesses decided advantages 

 over the present standard extract agar. 



No claim is made that the skimmed-milk powder media can 

 not be improved; we hope they can. As they stand at present, 

 however, any one of the three has advantages enough over the 

 extract agar to replace it as a standard medium for the deter- 

 mination of bacteria in milk, because: first, they show higher 

 counts than the standard extract agar and therefore counts 



