SECTION V. 
APPLIED BACTERIOLOGY. 
CHAPTER XXX. 
BACTERIOLOGY OF INIILK. 
A VERITABLE river of milk, collected from many sources, flows 
daily into the large cities of the country. ]\rilk 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 
growth of bacteria ordinarily not regarded as pathogenic may induce 
changes in this medium which render it unfit or even harmful for 
human use. If these changes are not of sufficient magnitude to alter 
