420 MILK 



made and an account published by Conn, who acted as referee. 

 A few of the most important comments made are these: 



"1. The Standard Methods for Bacterial Examination of 

 Milk published by the American Public Health Association need 

 revision, as they lay great emphasis on some of the least important 

 points, while they neglect to lay emphasis on some of the most 

 important points. 



"2. Individual analyses under the best conditions are subject 

 to considerable variation. 



"3. The question of the exact composition of the media to be 

 used is of far less significance than of the methods used in ma- 

 nipulation. 



"4. Greater care should be given to the proper dilution selected 

 for counting colonies and method of counting. Magnifying lenses 

 should be of uniform power. 



"5. A series of tests has proved that if a sample of milk can be 

 put into iced water, containing floating ice, it may be kept for 

 twenty hours with very little change in bacteria count." 



Breed and Stocking, on the other hand, have found that in 

 the hands of careful manipulators, using technic which differs 

 much in detail, the agar plate method has given very consistent 

 results when compared with the rapid work of laboratory assist- 

 ants using routine methods. 



It is obvious that the plate method, or any other method for 

 that matter, requires care, accuracy, and experience if con- 

 sistent results are to be obtained. The rush of work in com- 

 mercial laboratories and sometimes the lack of experienced work- 

 ers may explain in part the divergence of results. This diver- 

 gence is particularly unfortunate, since it tends to throw dis- 

 credit on bacterial counts in milk and renders their application 

 for improvement of milk-supplies more or less difficult. 



Frost has published a method for preparing "little plates" 

 and counting the colonies by means of the microscope within three 

 or four hours for raw milk and eight to twelve hours for pas- 

 teurized or very good milk. The author describes the method as 

 follows : 



"One-twentieth c.c. of milk is mixed with standard nutrient 

 agar and spread over a definite area of a sterile glass slide. When 

 the agar is hard, this little plate culture is put in the incubator 

 for about six hours under conditions which prevent evaporation. 

 It is then dried, given a preliminary treatment to prevent the 

 agar from firmly binding the stain, stained, decolorized, and 

 cleared. When this dried and stained plate culture is viewed 

 under the microscope the little colonies are definitely stained and 

 appear highly colored on a colorless or slightly colored background. 



