SECTION V. 



APPLIED BACTERIOLOGY. 



CHAPTER XXXI. 

 BACTERIOLOGY OF MILK. 



A VERITABLE river of milk, collected from many sources, flows 

 daily into the larger cities of the country. Milk is an important food, 

 particularly for infants and children, partly because it is relatively 

 inexpensive and requires little or no preliminary preparation, chiefly 

 because it contains in a small volume, all the essential nutritive 

 elements combined in readily utilizable form. Herein lies its potential 

 danger. It is a good culture medium for bacteria and its opacity pre- 

 cludes the possibility of visually detecting the contamination. Indeed, 

 considerable amounts of dirt and filth may be introduced into milk 

 without visibly changing its normal appearance. 



It is inevitable, from existing conditions, that milk from many 

 sources must be mixed before it appears in the open market; there 

 may be an element of danger or a measure of safety in this homo- 

 genizing process. If milk from a single dairy happens to be infected 

 with pathogenic bacteria, the degree of infection may be sufficient to 

 effectively seed the entire volume with which it is mingled, or the 

 degree of dilution may reduce the numbers of bacteria per volume 

 below the danger point of infection for man. 



The various manipulations to which milk is necessarily subjected 

 before it reaches the consumer afford ample opportunity for bacterial 

 contamination and the time which necessarily elapses between pro- 

 duction and consumption furnishes one of the additional elements 

 necessary for the development of adventitious bacteria. The tem- 

 perature at which the milk is maintained is another important physical 

 element which determines the extent of bacterial growth in it. 



A moderate number of bacteria pathogenic for man may lead to 

 infection of those who drink milk containing them, even if no develop- 

 ment of these organisms has taken place. On the other hand, the 



