THE RELATION OF MICROORGANISMS TO MILK. 295 



The immense numbers of bacteria found in milk in the large cities 

 are usually the result of the rapid growth of the Bact. lactis acidi group 

 resulting from the age of the milk and trie temperature at which it has 

 been kept. Such milk may also contain large numbers of those sapro- 

 phytic organisms which occur in the atmosphere and about the stables and 

 milk-house. The number of this group depends largely upon the sani- 

 tary conditions of production and the initial contamination. In ordinary 

 milk organisms of the Bact. lactis acidi type will constitute a very large 

 percentage of those present when the milk reaches the city even before 

 it shows any perceptible signs of souring. During the past few years 

 great progress has been made in the production of clean milk, and at 

 present quite an important part of the general milk supply of our cities 

 has a very much lower germ content than it had a few years ago. 



SPECIAL MILKS. In this class may be considered those milks known 

 as Selected, Inspected, or Guaranteed. As commonly used these terms 

 mean milk which has been produced and handled with considerably 

 more care than ordinary market milk but not with the extreme care 

 required for certified milk. Guaranteed milk is produced by herds which 

 have been shown by the tuberculin test to be free from tuberculosis. 

 Considerable care is exercised in all the operations of handling the milk. 

 The result is that these milks usually have a much lower germ content 

 than the ordinary milk supply of the same city. Sometimes the germ 

 content of such milk compares favorably with that of certified milk. 

 These milks may contain various types of normal milk organisms but 

 they should not contain any tubercle bacteria. 



CERTIFIED MILK. Certified milk means milk which has been produced 

 according to the regulations of and under the supervision of a medical 

 milk commission. The stables and cows are kept extremely clean. No 

 dust is allowed in the stable at milking time. The cow's flanks and udder 

 are washed just before milking, the milkers wear white suits and wash 

 their hands before milking each cow. Small-top pails are used and the 

 milk is cooled as soon as drawn from the cow. The extreme care ex- 

 ercised in the production and handling of this milk has a very marked 

 effect on the number of bacteria found in it. The following counts are 

 typical of certified milk. 



