62 Dairy Bacteriology. 



CHAPTER IV. 



INFECTION OF MILK WITH PATHOGENIC 

 BACTERIA. 



That the disease-producing, or pathogenic bacteria, 

 are able to infect milk supplies is shown by the fact 

 that numerous epidemics of contagious disease have 

 been directly traced to milk infection. Milk is gener- 

 ally consumed in a raw state, and as a considerable 

 number of this class of organisms are able not only to 

 live but actually grow in milk, which is such an ideal 

 culture-medium for the development of most bacteria, 

 it is not surprising that disease processes should be 

 traced to this source. The organisms in milk capable 

 of causing disease do not alter or change its physical 

 properties sufficiently to enable their presence to be 

 detected by a physical examination. 



Origin of pathogenic bacteria in milk. Disease-pro- 

 ducing bacteria may be grouped, with reference to their 

 relation toward milk, into two classes, depending upon 

 the manner in which infection occurs : 



Class I. Disease-producing bacteria capable of being 

 transmitted directly from a diseased animal to man 

 through the medium of infected milk. 



Class II. Bacteria pathogenic for man but not for 

 cattle, which are capable of thriving in milk after it is 

 drawn from the animal. 



In the first group, the disease produced by the speci- 

 fic organism must be common to both cattle and man. 



