404 MICROBIOLOGY OP MILK AND MILK PRODUCTS 



milk as a means of determining its purity. The methods used for this 

 purpose have followed quite closely those of the water bacteriologists.. 



For many years, dairy bacteriologists have endeavored to deterniine the number 

 of organisms in milk by plating it into nutrient agar or gelatin. By this method 

 the number of colonies developing in the plates is assumed to represent the germ 

 content of the milk. But even when the best methods are employed, the plate count 

 represents only the approximate, and not the exact number of bacteria in any lot Of 

 milk. It should also be borne in mind that such counts are always underestimates, 

 because of the fact that not all species will develop in any given medium or incubation 

 temperature. The careful worker can recognize certain types of bacteria in plain 

 media, but the addition of blue litmus solution to either agar or gelatin, greatly 

 assists in the differentiation of types and species. 



The Direct Microscopic Method. — The plating method is expensive because 

 of the large amount of time and materials needed. It is not possible for one person 

 to handle a large number of samples at one time. In routine work in the city labora- 

 tories this labor has been a serious drawback to this method. In order to decrease 

 the labor and give greater possibilities to the work Stewart devised a method by 

 which the bacterial condirion of milk can be studied by direct microscopic examina- 

 tion. His purpose was to determine only the species present, but later Slack and 

 still more recently Breed developed the method for determining the approximate 

 numbers as well as the general species present in a given sample of milk. 



Leucocytes. — The microscopic examination of milk sediment revealed the fact 

 that frequently a sample would be found which showed the presence of leucocytes in 

 greater or less numbers. The presence of these cells was regarded as important 

 because it was assumed that they showed the presence of inflammation and pus for- 

 mation in the udders of the cows producing the milk. 



Several methods have been used for determining the leucocyte content of milk. 

 "The Smear Sediment" and "Blood Counter" are methods which more strictly 

 beloiig to laboratory practices and will not be considered in this place. 



Bacteriological Milk Standards 



The relation of the bacterial content of milk to its wholesomeness 

 has led to the adoption of certain standards by the boards of health in 

 our cities. These standards recognize the fact that the germ content of 

 milk in the large cities is greater than in the smaller ones because of the 

 greater distance from which it is shipped and its age on arrival to the 

 city. New York City in 1900 adopted a maximum limit of 1,000,000 

 per c.c. Later Boston estabUshed a limit of 500,000 Chicago 1,000,000 

 from May to September, inclusive, and 500,000 from October to April, 

 inclusive and Rochester 100,000. Other cities have made similar 

 standards. 



