602 BACTERIOLOGY OF MILK 



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 

 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 possi- 

 bility of contamination from human sources, provides a fruitful source 

 of contamination with saprophytic organisms. The sterilization of 

 machinery of this type is difficult to accomplish and not infrequently 

 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. 



