454 BACTERIA PATHOGENIC TO MAN 



From the milks altogether, 239 varieties of bacteria were isolated 

 and studied. These 239 varieties, having some cultural or other differ- 

 ences, were divided into the 31 classes, each class containing from 1 

 to 39 more or less closely related organisms. 



As to the sources of bacteria found in milk, we made sufficient experi- 

 ments to satisfy us that they came chiefly from outside the udder and 

 milk-ducts. 



Bacteria were isolated from various materials which under certain 

 conditions might be sources of contamination for the milk, and the 

 cultures compared with those taken from milk. Thus there were 

 obtained from 20 specimens of hay and grass, 31 varieties of bacteria; 

 from 15 specimens of feces, manure, and intestinal contents, 28 varieties; 

 from 10 specimens of feed, 17 varieties. Of these 76 varieties there 

 were 26 which resembled closely those from milk viz., 11 from grass 

 or hay; 26 from manure; 5 from feed. 



During the investigation a number of the varieties isolated from 

 milk were shown to be identical with types commonly found in water. 



From the few facts quoted above and from many other observations 

 made during the course of the work, it would seem that the term "milk 

 bacteria" assumes a condition which does not exist in fact. The expres- 

 sion would seem to indicate that a few varieties, especially those derived 

 in some way from the cow, are commonly found in milk, which forms 

 having entered the milk while still in the udder, or after its withdrawal, 

 are so well fitted to develop in milk that they overgrow all other varieties. 



As a matter of fact, it was found that milk taken from a number of 

 cows, in which almost no outside contamination had occurred, and 

 plated immediately, contained, as a rule, very few bacteria, and these 

 were streptococci, staphylococci, and other varieties of bacteria not 

 often found in milk sold in New York City; the temperature at which 

 milk is kept being less suitable for them than for the bacteria which fall 

 into the milk from dust, manure, etc. A number of specimens of fairly 

 fresh market milk averaging 200,000 bacteria per cubic centimetre were 

 examined immediately, and again after twelve to twenty-four hours. In 

 almost every test the three or four predominant varieties of the fresher 

 milk remained as the predominant varieties after the period mentioned. 



The above experiments seem to show that organisms which have 

 gained a good percentage in the ordinary commercial milk at time of 

 sale will be likely to hold the same relative place for as long a period 

 as milk is usually kept. After the bacteria pass the ten or twenty 

 million mark a change occurs, since the increasing acidity inhibits the 

 growth of some forms before it does that of others. Thus some varieties 

 of the lactic acid bacteria can increase until the acidity is twice as great 

 as that which inhibits the growth of streptococci. Before milk reaches 

 the curdling point, the bacteria have usually reached over a billion to 

 each cubic centimetre. For the most part specimens of milk from different 

 localities showed a difference in the character of the bacteria present, in 

 the same way that the bacteria from hay, feed, etc., varied. Even the 

 intestinal contents of cows, the bacteriology of which might be expected 



