THE RELATION OF MICROORGANISMS TO MILK 467 



This relation does not always hold for the reason that a gram of 

 one kind of dirt may contain infinitely more organisms than an equal 

 amount of some other kind. The difference in the solubility of various 

 forms of dirt always causes apparent discrepancies in this normal 

 relation. In the majority of cases, however, the relation shown in 

 the above examples will hold reasonably true in the case of fresh milk. 

 There is also a general relation between the number of bacteria in 

 fresh milk and the length of time it will keep before souring and curd- 

 ling. In this case the relation is in inverse ratio, the smaller the initial 

 contamination, the longer the keeping time, and vice versa. This re- 

 lation is also shown in the table given above. There are many ir- 

 regularities, however, in this relation because of differences in the 

 flora of fresh milk. It may frequently happen that a sample of milk 

 containing a relatively high number of organisms will not sour as 

 quickly as another sample with a smaller original germ content. The 

 associative action of the different species of organisms is an important 

 factor here. In making comparisons of this sort, it is, of course, 

 necessary that the different samples be held at the same temperatures. 



MILK AS A CARRIER OF DISEASE-PRODUCING ORGANISMS 



It is not the purpose of this chapter to discuss in detail the diseases 

 which may be carried by milk, but a chapter on bacteriology of milk 

 would be incomplete without a brief discussion of this important 

 subject. 



From the standpoint of their relation to the health of the con- 

 sumer the microorganisms in milk may be divided into three groups 

 on the basis of whether they are beneficial, inert or injurious to health. 



Acid Forms. -The preservative properties of sour milk have been 

 known since very ancient times. Its use as a preservative for meat, 

 eggs and other perishable food products demonstrates the value of 

 sour milk as a means of preventing decomposition. It has also been 

 known for a long time that sour milk has a certain therapeutic value 

 because of the action of the lactic bacteria in preventing harmful 

 fermentations in the digestive tract. More recently the work of 

 Metchnikoff has shown the usefulness of sour milk both for the treat- 

 ment and prevention of intestinal disorders by inhibiting the develop- 

 ment of the putrefactive bacteria in the digestive tract. In view of 

 the value of sour milk for preventing certain forms of disease and its 



