CHAPTER XXX 



BACTERIA IN MILK 



ROBERT S. BREED 

 New York Agricultural Experiment Station, Geneva, N. Y. 



Milk is a secretion produced from the material contained in blood serum or lymph 

 by the glandular epithelial cells lining the alveoli of the mammary glands of mammals. 

 From the moment it is secreted until its final transformation into other forms of mat- 

 ter, it may be, and usually is, acted upon by bacteria. 



UDDER BACTERIA 

 HISTORY OF THEIR DISCOVERY 



Roberts,' in 1874, was apparently the first to demonstrate that, with aseptic pre- 

 cautions, small portions of sterile milk could be drawn from the udder. Because his 

 work has been frequently overlooked, it may be well to quote his description (p. 469) : 



From a test-tube thus filled I charged ten empty sterilized tubes in the manner already 

 described, and resealed their capillary orifices. Of these ten, three remained unchanged. When 

 examined from three to six weeks afterwards the milk in them was perfectly sweet to the 

 taste, its reaction was neutral or faintly acid, like that of fresh milk, there was no curdling, 

 and no signs of organisms under the microscope. The other seven changed within ten or 

 twelve days. Some of them curdled and others putrefied; all became highly acid, and, under 

 the microscope, Bacteria, either staff-shaped or spherical, were found in them. 



A few years later Lister^ succeeded in securing two sterile tubes out of ten tubes 

 in a similar experiment carried out independently. Because of this and other work 

 showing that normal blood, urine, and similar body fluids were sterile, it was generally 

 believed for ten to fifteen years after this that milk as drawn from the udder is sterile. 

 The organisms found in the tubes which did not remain sterile were regarded as air 

 contaminations. 



Schulz,^ under the direction of K. B. Lehmann in i8gi, showed that the first milk 

 drawn contained large numbers of bacteria, while milk drawn midway and at the 

 close of the milking process contained fewer and fewer bacteria. Moore'' soon after 

 also used the agar-plate technique to study the number of bacteria in milk as drawn 

 from the udder. He reported that 



freshly drawn fore milk contains a variable number of bacteria, varying in number from a 

 few individuals to many thousand per cubic centimeter. These are distributed among sev- 

 eral species. The last milk drawn at a regular milking contains, as compared with the fore 

 milk, very few micro-organisms. It is the exception, however, to find a sample of milk that is 

 free from micro-organisms unless it is taken during the latter part of the milking process 

 from a single quarter of the udder. 



' Roberts, W.: Phil. Tr. Roy. Soc, London, 164, 457. 1874. 



^Lister, J.: Quart. J. Micr. Sc, 18, 177. 1874; Tr. Path. Soc, London, 29, 425. 1S78. 

 5 Schulz, L.: Arch.f. Hyg., 14, 260. 1892. 



"I Moore, V. A.: U.S. Dept. Agric. Ann. Rep. Bur. An. Ind., 12 and 13, 261. 1897. 



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