THE DISTRIBUTION OF MICRO-ORGANISMS 189 



MICRO-ORGANISMS OF FOOD. 

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Milk. Milk is the natural food of young mammals, and 

 naturally it is taken directly from the mammary gland into 

 the digestive tract of the young mammal. For many centuries, 

 however, the milk of certain animals has been extensively used 

 as a commercial food for man. The principal animals furnishing 

 commercial milk are the cow, goat and mare. The chemical 

 composition of milk is different in different animals, in the same 

 animal at different periods of lactation, and even that obtained 

 at different stages of a single milking shows considerable varia- 

 tion. In general cow's milk has the following composition. 



Variation. Average. 



Fat 3-6 4 per cent. 



Lactose 1-3 2 per cent. 



Protein 5-8 7 per cent. 



Water 84-88 87 per cent. 



It is an excellent medium for the growth of most bacteria and is 

 commonly used in the laboratory for this purpose. 



There are about 200 species and varieties of bacteria which 

 commonly occur in milk. They are derived in part from the 

 udder itself. Bacteria are always present in the milk ducts of 

 the udder and are fairly abundant in the first portions of milk 

 drawn, so that milk very carefully drawn from healthy animals 

 may contain 200 to 400 bacteria per cubic centimeter. Milk 

 from diseased udders may be very rich in pathogenic micro- 

 organisms. As the milk is drawn, many micro-organisms usually 

 gain entrance to it from the atmosphere, the hands of the milker 

 and the utensils with which it comes in contact. From the 

 body of the cow, particles of dust and hairs drop into the milk, 

 carrying with them the flora of the intestine and of the skin of 

 the cow. From the milker, the material on the hands and possibly 

 also from the nose and mouth may reach the milk. The utensils, 

 unless sterilized before use, contribute the microbic flora of the 

 previous milkings, of the water used for cleansing and from the 



