COMPARATIVE GROWTH OF BACTERIA. 103 



shows, therefore, a constant change in percentage of the dif- 

 ferent species. During the three days following the milking 

 there is a struggle of the different species of bacteria in the 

 milk with each other, and the results of such a complicated 

 struggle cannot be predicted with any degree of certainty. 

 The action of the milk bacteria upon the milk, and still more 

 upon cream in the cream ripening, will be largely dependent 

 upon the results of this contest, and the question whether 

 favorable results are obtained in cream ripening will depend 

 not wholly upon the numbers and actual kinds of bacteria 

 present at the start. It is dependent even more upon .which 

 of them develop in abundance and which are prevented from 

 growing by the conditions of the struggle. While the results 

 of this complicated interaction of bacteria can never be 

 predicted with certainty the usual course of this growth is as 

 follows : 



The general growth of the bacteria in normal milk may be 

 divided into two stages. While the two stages are not sharply 

 separated it is more easy to understand the whole process by 

 recognizing these two periods. The first period is the time 

 from the start up to the .time when the milk has about 10,000,- 

 ooo bacteria per c.c. The second is from this point until the 

 souring and curdling of the milk. The first period thus 

 includes the whole time before the milk is commonly dis- 

 tributed to the consumer, at least in cases where the milk is 

 delivered fresh. The second concerns the later changes of 

 the milk in the consumer's possession and those which effect 

 cream ripening (101). 



First Period. At the outset the number of bacteria is not 

 very high, but the variety is likely to be considerable. A 

 sample of milk from a single cow is likely to have eight to a 

 dozen or even more species of bacteria, each present in vary- 

 ing numbers, the number of each species depending of course 

 upon the extent of the original contamination. Commonly 

 the larger number of these milk bacteria are those which 



