BACTERIOLOGY OF COW'S MILK 285 



nitrifying bacteria, and add to the nutrition of the plant by adding 

 to it ammonia. 



Bacteriology of Cow's Milk 



Theoretically the milk in the interior of the breasts of nursing 

 women and the udders of cows is sterile. So soon as it leaves the 

 nipple it becomes contaminated with bacteria, and by the time it 

 reaches the pail, in the case of cow's milk, it is far from sterile. 



Bacteria of the air, and dust from the cattle and bedding, at 

 every movement of the cow, and by the agency of flies, find their 

 way into milk and contaminate it. The number of bacteria that 

 develops in the milk depends upon the number that reach it in the 

 first place, the temperature of the air, and the length of time milk 

 is kept at a temperature favorable for their multiplication. Two 

 hundred and thirty-nine different varieties of bacteria have been 

 isolated from milk at different times. 



Pathogenic varieties of bacteria that have been found in cow's 

 milk include- the tubercle bacillus, Streptococcus pyogenes, Staphy- 

 lococcus aureus, the colon bacillus, typhoid bacillus, the diphtheria 

 bacillus, and a whole host of bacteria that sour or ferment the 

 milk and render it unwholesome or poisonous for young children. 



Cattle may be tuberculous, and the tubercle bacilli may reach 

 the milk in this way. There may be abscesses of the udder and 

 the streptococci from the pus may cause infection in those that 

 use it. Ordinary follicular tonsillitis may be caused in this way. 



Bacteria may develop rapidly in milk, which is a good culture 

 medium, until they number many millions per cubic centimeter 

 (sometimes 200,000,000). 



In good milk the number of bacteria may increase when the 

 temperature is goF., from 5,200 originally in the milk imme- 

 diately after milking, to 654,000 in eight hours. 



By exposing milk to a temperature of i65F. for twenty to 

 thirty minutes and quickly cooling (Pasteurization) most of the 

 non-spore-bearing bacteria are destroyed, so that the number may 



