684 BACTERIOLOGY OF MILK 
the physical appearance of the milk, or if they are not perceptible to 
the senses, they may easily escape detection and yet lead to illness 
of the consumer. It is obvious, therefore, that those very elements 
which make milk a valuable food create conditions, themselves innocu- 
ous, through which it may become actively or passively a vehicle for 
the transmission of disease to man. 
One of the great hygienic problems of the present time is that of 
maintaining and safeguarding the milk supply. 
Sources of Bacterial Contamination of Milk.— Milk freshly drawn 
from the udder of a healthy cow, although practically never sterile, 
rarely contains many bacteria. The greatest contamination of milk 
probably takes place from unsterile utensils, although undoubtedly 
unclean animals, filthy surroundings and dusty air contribute many 
bacteria to it. Organisms introduced into milk from the hands of the 
milker and from his respiratory tract may be far more formidable to 
the consumer than mere numbers of saprophytic bacteria. 
The ever-increasing application of complicated machinery for 
handling and bottling milk, while reducing to a large degree the 
possibility of contamhiation from human sources, provides a fruitful 
source of contamination with saprophytic organisms. ^ The steriliza- 
tion of machinery of this type is difficult to accomplish and not infre- 
quently incomplete cleansing between periods of actual use leaves a 
residuum of fluid sufficiently rich in nutritive substances to permit of 
extensive bacterial development. The first portion of milk run through 
a machine in this condition must inevitably be grossly seeded with 
microorganisms. 
The development of bacteria which have gained entrance to milk 
depends to a very considerable degree upon the temperature at which 
the milk is kept and the time which elapses between production and 
consumption. 
Estimation of the Bacterial Content of Milk.— It is obvious from 
the preceding observations that adventitious milk bacteria may be 
harmful to man either because they are pathogenic or because they 
produce changes in the composition of milk which make it unfit for 
human consumption. From an hygienic point of view, therefore, 
milk offered for sale should be free from pathogenic microorganisms 
and of low bacterial content. 
The numbers of bacteria in milk are determined in practice by two 
distinct methods : 
(a) The numbers of organisms which will grow upon ordinary 
laboratory media, as nutrient agar (cultural count), and: 
(6) By direct microscopic count. 
(a) Cultural Count. Method: 1 cc. of a well-mixed sample of 
milk is diluted j^-^, yoVo» T^ioTr or even TTTToTFir with sterile water, 
depending upon the grade of the sample, and plated on nutrient 
• Harding: Jour. Dairy Service, July, 1920, vol. 3. 
