BACTERIOLOGICAL STUDY OF MILK 633 



bacterial standard for approved milk of 10,000 bacteria to 

 the cubic centimeter. Experience has shown that it is 

 possible to market milk that meets this bacterial standard 

 sometimes with merely ordinary precautions with regard 

 to cleanliness. In larger dairies it has frequently been a 

 question of some difficulty on account of the elaborate 

 scale on which the business is conducted. 



Quantitative Bacteriological Analysis. In the quantitative 

 bacteriological examination of market milk it is necessary 

 to dilute the milk with sterile water or sterile salt solution 

 before plating on account of the very large numbers of 

 bacteria present. The degree of dilution that is necessary 

 will depend upon the nature of the dairy from which the 

 milk is derived, the age of the milk, and the temperature 

 at which it has been kept. Usually a dilution of 1 to 100, 

 1 to 1000, and 1 to 10,000 is sufficient. From these dilutions 

 plate cultures are made with 0.1, 0.2, 0.3 cubic centimeter 

 of each dilution. 



Qualitative Bacteriological Analysis. Aside from the 

 quantitative bacteriological analysis of milk the qualita- 

 tive analysis has received a great deal of attention. Detailed 

 qualitative analysis necessarily entails an enormous amount 

 of labor, but the detection of certain forms of bacteria is 

 not always very difficult. This applies especially to the 

 detection of streptococci. 



Since milk containing streptococci in considerable num- 

 bers is derived from the udder of a cow suffering from some 

 form of mastitis, it is always possible to find pus in such 

 milk. Consequently it is customary to examine such milk 

 for the presence of both streptococci and pus. This is done 

 by centrifuging a cubic centimeter of the milk and collecting 

 the sediment on a clean cover-slip and staining with Loffler's 



