SOURCES OF COMMON MILK BACTERIA 67 



of contamination. If milking takes place immediately after 

 feeding, the number of bacteria from this source is very high. 

 These facts are easily demonstrated by simple tests. The air 

 of the dairy and the out-of-door air to which the milk may be 

 exposed are also sources of some contamination, but these are 

 usually slight. One curious instance has been recorded of milk 

 being filled with troublesome organisms from the open cans 

 standing under trees on the leaves of which the organism was 

 growing. The bacteria derived from the air are, as a rule, 

 harmless forms. Bact. lactis acidi is undoubtedly in many cases 

 derived from this source. We do not positively know the orig- 

 inal source of this germ, although Barthel has found it upon 

 the leaves of growing plants, whence it might easily get into 

 hay. 1 But we do know that it gets into the milk from the dust 

 and the air of the milking stable, and from the dirt attached to 

 the skin of the cows, from faeces and indeed from almost all 

 objects in the barn. Bact. aero genes probably also comes from 

 the air. Other bacteria from this source are representatives 

 of the liquefying type and tend to produce liquefaction. Many 

 neutral bacteria are also found. 



Milk Vessels. Perhaps the most prolific source of milk con- 

 tamination, so far as numbers are concerned, is found in the 

 bacteria which remain alive in the milk utensils between one 

 milking and the next. Bacteria are not usually killed by short 

 drying. After each milking the milking pails and other utensils 

 are commonly washed, sometimes, indeed, thoroughly washed 

 with hot water and soap; but no such washing is sufficient to 

 destroy the bacteria which may be in the pails. No simple 

 washing will make a milk pail bacteriologically clean. After 

 any ordinary washing there will be left clinging to such utensils 

 many bacteria which are ready to develop as soon as fresh milk 

 is added to furnish them with proper food and proper condi- 



1 Barthel. Rev. Gen. d'Lait, V., p. 223, 1906. 

 Burri. Cent. f. Bact., II., x., p. 756, 1903. 



