162 BACTEEIA IN MILK 



With regard to the last, however, the difficulty of differentiating 

 harmless from harmful forms constitutes at present a serious 

 factor 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. Savage is of 

 opinion that milk from the cow ought not to contain more than 

 one bacillus coli per c.c. ; on this basis vended milk containing 

 from 100 to 1000 per c.c. might be accepted as satisfactory. 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, 

 it is probable that in many cases the same organism has passed 

 under a number of different* names. 



There is a general agreement that two main types occur. The first of 

 these is the streptococcus lacticus, originally described by Kruse. This is 



