134 BACTERIOLOGY OF WATER, AIR, MILK, ETC. 



twenty-four hours and from such tubes plate out on milk serum agar (coagulate 

 boiling milk with a few drops of acetic acid, filter and add i% peptone, 2% glucose 

 and 1.5% agar). 



As they grow in very acid media the term acidophilous is applied. It was sup- 

 posed that these bacteria were peculiar to certain fermented milks as matzoon and 

 yogurt. Hastings has shown the group to be present in milk in the United States 

 and considers the source to be the alimentary tract of cows. 



Another source of information as to the quality of a milk may be 

 derived from a study of the number of leukocytes or pus cells contained 

 in i c.c. of the milk. It must be understood that cellular elements 

 which differ only slightly from true pus cells may be found in the milk 

 of healthy cows and may be found in great numbers. Statements have 

 been made that such cells are neither amoeboid nor phagocytic. 



The Doane-Buckley method is probably the most accurate. In this you throw 

 down the cellular contents of 10 c.c. of milk in a centrifuge revolving about 1000 

 times a minute for ten to twenty minutes. Then remove supernatant milk and add 

 0.5 c.c. of Toisson's' solution to the sediment. You thus have the leukocytes of 

 10 c.c. contained in 0.5 c.c. (Concentrated twenty times.) Make a haematocytom- 

 eter preparation as for blood and find the average munber of cells for each square 

 millimeter. Then multiply this by 10 to get the number of cells in a cubic millimeter. 

 As a cubic millimeter is one thousand times smaller than a cubic centimeter, you 

 multiply the number per cubic millimeter by 1000. Then, as the milk was concen- 

 trated twenty times, you divide by 20. (If it were diluted twenty times, you would 

 multiply by 20.) 



Example. Found an average of 50 cells per square millimeter. This would 

 make 500 per cubic millimeter, and 500,000 per c.c.; then 500,000 divided by 20 

 would give 25,000. 



There is no agreement as to a standard for allowable leukocytes. Even in 

 apparently healthy animals they may exceed 100,000 per c.c. Doane has suggested 

 500,000 per c.c. as a preferable limit. 



The smear methods for determining the number of leukocytes present do not 

 compare in accuracy with the volumetric ones. 



To summarize, we may state that the bacterial count is an indicator 

 of the care used in handling the milk while the presence of harmful 

 bacteria (qualitative examination) or numerous pus cells indicates 

 disease in the cow. During 1912 severe epidemics of sore throat due 

 to a streptococcus, S. epidemicus, were traced to milk of cows having 

 probably suffered from mastitis. In Baltimore the milk had been 

 pasteurized by the flash method which indicates the unreliability of this 

 process. 



Pasteurization of Milk. The objections to this method of preserving milk 

 have been(i) that the lactic acid bacteria which have been by some credited with 



