iv BEHAVIOUR OF BACTERIA IN MILK 51 



termining the species of bacteria which grow in milk." The 

 inilk used for the investigation was ordinary market milk, but 

 quite fresh, being only one or two hours old. It usually con- 

 tained about 20,000 bacteria per c.c. Two series of experi- 

 ments were carried out, the milk for the first series being 

 kept at 37, 20, and 10 C., and for the second at 20, 10, 

 and 1 C. The milk samples were bacterially examined at 

 intervals which varied with the temperatures employed. After 

 some preliminary tests, it was found that the best intervals 

 were as follows: Milk kept at 37 C. was plated every 2 

 hours, that at 20 C. every 6 hours, that at 10 C. every 12 

 hours, and that kept at 1 C. about every 3 days. The 

 culture medium used for making the plates was litmus lactose 

 whey gelatine. In the study of the plates, the total number 

 of bacteria was determined, and then the total number of 

 different types of colonies that could be differentiated from 

 each other by hand lens and low power of the microscope. 



It was found possible to distinguish, without much 

 difficulty, about 15 different groups of bacteria, that is, 15 

 different types of distinguishing colonies in litmus gelatine. 

 The groups tabulated are differentiated from each other wholly 

 by the character of their colonies upon litmus gelatine. All 

 the groups included several species of bacteria, and sometimes 

 indeed were quite complex, and the weak point of the research 

 is this rather indefinite grouping and differentiation. 



From the numerous experiments made, the following were 

 the main conclusions arrived at. 



(1) The effect of variations in temperature upon the develop- 

 ment of the different species of bacteria in milk is not always 

 the same under apparently identical conditions. In spite of 

 such variations, there appears to be clearly discernible a normal 

 development of bacteria associated with different temperatures. 



(2) There is, in all cases, a certain period at the beginning 

 when there is no increase in the total number of bacteria. 

 During this period, some species are multiplying, while others 

 are apparently dying. The length of this period depends 

 upon temperature. At 37 C. it is very short, while at 1 C. it 

 may last from 6 to 8 days, since at this temperature, milk may, 

 in 6 days, actually contain fewer bacteria than when fresh. 



(3) After this preliminary period, there always follows 



