138 BACTERIA IN MILK. 



or, in cases of inflamed udders, it is likely to contain pus, together 

 with considerable quantities of chain-forming streptococci. These 

 should not be present in good milk, and there is reason for believing 

 that they are the cause of certain illnesses in man. 



Confining our attention for the present to milk from healthy 

 animals, we notice that, if we could keep bacteria out of the milk, 

 none of the ordinary changes, not even the souring which is so 

 nearly universal in normal milk, would take place. Indeed, milk 

 which is free from bacteria will remain visibly unchanged for an 

 indefinite time. It is not, however, absolutely free from subsequent 

 chemical changes, since there is present in the milk an enzyme which 

 produces slow changes. This enzyme, called galactase, is secreted 

 by the milk gland with the milk, and may thus be said to be part of 

 the milk. It can slowly convert the casein of the milk into soluble 

 proteids. Its action is very slow, however, and seemingly of no 

 significance, except in the ripening of cheese. At all events, none 

 of the ordinary fermentations appearing in milk are attributed to 

 this galactase or to any other part of the milk itself, but are all due 

 to microorganisms. We may, therefore, take as a starting-point 

 these two highly important facts: i. Milk from healthy cows will, 

 if it could be kept free from bacteria, show none of the ordinary 

 milk fermentations. 2. All of these fermentations ^are due to 

 microorganisms that get into the milk after the milk is secreted from 

 the mammary gland. 



SOURCES OF MILK BACTERIA. 



Recognizing that milk is germ free when secreted from the milk 

 gland, we are hardly prepared to learn that, by the time it has been 

 drawn from the cow, received in the milk-pail, and removed from the 

 cow stall, it may contain bacteria to the extent of many thousands 

 per c.c. But this is frequently and, indeed, commonly the case. 

 The number of bacteria in freshly drawn milk varies greatly with 

 the conditions existing in the dairy. There may be only a few 

 hundreds in each c.c., or, under exceptional conditions, a smaller 

 number still; but it is much more likely that the milk, by the time 



