BACTERIA OF MILK 75 



Exercise 110. Quantitative Examination of tlie Organisms in Milk 



The enumeration of bacteria in milk, while in many respects 

 a simple undertaking, is full of difficulties if one attempts to 

 obtain accurate results. In freshly drawn milk the bacteria are 

 unevenly distributed, larger numbers being found in the froth, 

 on hairs, on fragments of manure, in crevices of the vessels, etc. 

 Even when the milk is thoroughly shaken, the distribution of the 

 bacteria is not usually uniform enough to give good counts. In 

 older milk, numbers of the bacteria are carried to the surface 

 layers as the fat globules rise. The lactic-acid-forming bacteria 

 begin to predominate as the acid which they produce kills off 

 their competitors. Then agaia the temperature employed and 

 the media commonly used for making plate cultures are not 

 suited for the development of all organisms in milk. 



In spite of these objections the bacteriological examination of 

 milk gives a fairly accurate picture of the conditions under 

 which the milk has been produced or handled, and continues to 

 be used as one of the important standards for the inspection of 

 commercial milk samples. 



1. Procure samples of freshly drawn milk, of raw market milk, 

 of pasteurized market milk, and of cream, having first thoroughly 

 shaken the vessel from which the sample is to be drawn. 



2. Dilute samples about as follows, with sterile physiological 

 salt solution, using the method employed for diluting water 

 samples (Exercise 74) : 



Freshly drawn milk, 1 : 100. 



Raw market milk, 1 : 1000 and 1 : 10,000. 



Pasteurized milk, 1 : 100. 



Cream, 1:100,000. 



3. Make plates with beef gelatin, Heyden-Nahrstoff agar, 

 dextrose-litmus agar, and whey agar. 



4. Determine the total number of colonies and the proportion 

 of acid-forming colonies. 



Note. This experiment may be varied ad libitum by counting colonies 

 on milk drawn from dlflerent sources, by different methods, and handled in 

 various ways. 



