NUMBER AND SOURCE OF BACTERIA IN MILK 261 



Several observers have noted that in many cases the 

 number of bacteria in milk decreases for a time after it is 

 drawn from the cow. The length of time during which 

 the decrease continues depends upon the kinds of 

 organisms present and the temperature at which the 

 milk is kept. When the temperature is low say, about 

 10 C. the decrease may continue for thirty-six hours 

 or more ; at 30 C. it lasts only for two or three hours. 

 This peculiar fall in numbers and' cessation of bacterial 

 development, which occurs only during the first few 

 hours in samples of fresh milk, has been attributed by 

 some to the presence in the milk of a bactericidal sub- 

 stance which is soon used up or decomposed after it has 

 left the udder. It may, however, be due to the death of 

 many of the organisms which have come from the udder 

 of the animal or from the surrounding air, owing to the 

 altered conditions of existence to which they are exposed ; 

 those unaccustomed to live in milk die off or take time 

 to become adapted to it. 



After the bacterial content has reached its lowest 

 point a steady rise sets in and proceeds more or less 

 rapidly, according to the temperature, until a maximum 

 is attained, about the time the milk becomes strongly 

 acid and curdled. From this point the numbers begin to 

 decrease again, and little by little the bacteria die off until 



