168 BACTERIA IN MILK 



of cleanliness of the dairy. Another organism which has a 

 similar significance is the b. enteritidis sporogenes, and along 

 with this may be associated the streptococci often found in 

 milk. With regard to the last, however, the difficulty of 

 differentiating harmless from harmful forms constitutes at 

 present an insuperable obstacle in determining the significance 

 to be attached to their presence. 



An endeavour is sometimes made to set up standards of 

 bacterial purity in milk, based on an enumeration by plating 

 methods of the bacteria present, but such standards are of little 

 practical value on account of the difficulties lying in the way of 

 their application. Thus the conditions of collection and dis- 

 tribution of every supply, seasonal variations in temperature, 

 etc., would require to be considered in determining the bacterial 

 content which would be consistent with the non-occurrence of 

 souring of the milk during the period between withdrawal from 

 the cow and consumption. Given a sufficient number of 

 properly conducted dairies, however, data to form a basis for 

 setting up standards of bacterial purity in milk might be 

 obtained. Thus the enumeration of a large series of samples 

 of milk from well-kept cows would furnish an idea of the degree 

 of bacterial contamination which is unavoidable, and a standard 

 for milk as it leaves the dairy might be obtained. On such 

 lines important observations have been made by Savage dealing 

 with the content of good milk in b. coli. According to these, 

 this organism is usually not present in greater quantities than 

 1 per c.c. As Savage points out, by studying the growth 

 conditions of b. coli in milk it might be possible to determine 

 whether a milk as it reaches the consumer has come from a clean 

 dairy. This is an example of what further inquiry might result 

 in. At present, however, the only practicable method of securing 

 a reasonably pure milk supply is to insist on cleanliness in the 

 dairy. 



The Souring of Milk. Under ordinary conditions the first 

 evidence of bacterial activity, and from the economic standpoint 

 the most important, is the occurrence of souring due to the 

 formation of lactic and other allied acids, and the action of 

 these on the albuminous constituents is one of the factors in 

 curdling. The subsequent changes vary with the bacteria 

 present, but ultimately these lead up to putrefaction of the 

 ordinary type. The importance of the souring of milk has 

 caused much attention to be devoted to the process, and an 

 enormous number of bacteria has by various observers been 

 described. While various organisms are undoubtedly concerned, 



