382 BACTERIA IN MILK 



Large numbers of these cells occur in milk that has every appearance of being normal 

 (Breed).' Other cellular debris, such as fat-laden gland cells (colostral corpuscles), 

 gland-cell nuclei, and cells from the lining of the ducts, also occur in normal milk. 

 This cellular material usually occurs in abundance only when conditions are abnormal, 

 as at the beginning and end of lactation. There is no apparent relationship between 

 this epithelial-cell debris and bacterial infections of the udder, and it seems to repre- 

 sent normal wastage of worn-out cells. 



BACTERIA ADDED TO MILK FROM EXTRANEOUS SOURCES 



As soon as milk is drawn from the udder, it becomes contaminated with bacteria 

 from extraneous sources. These may be washed from the surface of pails, milking- 

 machine tubes, milker pails, strainers, surface coolers, milk cans, and the like, or they 

 may drop into the milk with floating dust or particles of dirt from the flanks of the 

 cow and the milker's clothing. Extensive studies' have been made to determine the 

 number of bacteria ordinarily found in each of these habitats in order to give practical 

 dairymen and others an idea where to search for contaminations when the number of 

 bacteria in market milk is in excess of the number which might normally be expected. 

 Poorly cleaned, moist utensils that are allowed to stand at temperatures favorable for 

 the development of bacteria have been found to be the most important source of these 

 extraneous bacteria. Under American dairy conditions the two utensils that most 

 frequently cause trouble are milk cans and the rubber tubing of milking machines. 

 Routine laboratory methods used for the control of the sanitary quality of market 

 milk have been standardized by the Laboratory Section of the American Public 

 Health Association.^ 



GERMICIDAL ACTION OF MILK 



It is generally agreed"" that fresh milk may contain certain substances derived from 

 the blood that have a germicidal action on bacteria, so that the number of bacteria 

 present in fresh milk usually decreases for a few hours after the milk is drawn. The 

 effectiveness of this action varies with the milk of individual cows, and some claim 

 that the germicidal action may be supplemented by a certain lag in the growth curve 

 produced by the introduction of bacteria in all stages of vitality into a new environ- 

 ment. This delay in the increase in the number of bacteria is utilized in a practical 

 way in the dairy industry where regulations permit dairymen to bring uncooled milk 

 to milk plants if this is delivered within four hours after the milk is drawn. 



TYPES OF SAPROPHYTIC BACTERIA IN MILK 

 THE BLUE-MILK ORGANISM 



Historically, the first species of saprophytic bacteria to attract attention in milk 

 was the one which causes a blue color in milk that has developed a certain acidity. 



■ Breed, R. S.: ibid., Bull. j8o. 1914. 

 2 Breed, R. S.: ibid., Circ. pj. 1927. 



^ Laboratory Section of the American Public Health Association: Standard Methods of Milk 

 Analysis (5th ed.). 1927. 



« Jones, F. E., and Little, R. B.: J. Exper. Med., 45, 319. 1927. 



