GROWTH OF BACTERIA IN NORMAL MILK. I 7 



it can be done by any other method yet described. It has 

 required several years for us to learn to understand the results 

 So as to work with a considerable degree of satisfaction, and 

 even yet we are only able to get an approximation toward a 

 complete analysis. Experience, however, has enabled us to 

 distinguish with considerable accuracy the colonies produced 

 upon gelatin plates by all, or at least most, of the more com- 

 mon milk bacteria. 



METHOD OF EXPERIMENTATION. 



The method that has been adopted in this laboratory is based 

 upon the use of a gelatin culture medium to which has been 

 added a certain amount of litmus. This culture medium has 

 already been described, but its careful preparation is so impor- 

 tant for a proper differentiation of species that a description of 

 the method of preparing it is here given. 



Preparation of litmus-s2igar-gelatin. — The culture medium 

 which we use is made as follows: 



I. Water, - - - 500 cc. 

 Peptone, - - - 10 grams. 

 Milk sugar, - - 30 grams. 

 Gelatin, - - - 120 grams. 

 L,iebig's extract of beef, 5 grams. 



The amount of milk sugar added is about 3 per cent, of the 

 solution finally prepared. The purpose of this is to obtain as 

 closely as practicable the conditions in milk. We use Liebig's 

 extract of beef rather than the juice of chopped beef, because 

 our experience has indicated that, for our purpose, this is per- 

 fectly satisfactory, and its use is much simpler, requiring less 

 time and trouble. 



The materials are placed together in a dish and dissolved by 

 heating at a temperature below 60° C. to make a solution 

 which, as will be seen, contains twice the quantity of the 

 various ingredients that is contained in the ordinary gelatin 

 culture media. After the material has become thoroughly dis- 

 solved we neutralize the solution directly, by use of NaOH solu- 

 tion, testing the material for neutralization with litmus rather 

 than phenolthalein. We use red and blue litmus papers made 

 in our laboratory and which are especially delicate, and we add 



