Il6 BACTERIOLOGY OF WATER, AIR, MILK ; ETC. 



75 colonies. Therefore the milk contained in each c.c. 75 x 2 x 20,000 

 (dilution) = 3,000,000 the number of bacteria in each c.c. of milk. 



Lactose litmus gelatin or agar is to be preferred in milk-work, as 

 the normal lactic acid bacteria produce reddish colonies which are very 

 striking. A standard easily attained for high-grade, certified milk 

 would be 5,000 to 10,000 per c.c. 



In the qualitative examination of milk, many dairies employ the 

 fermentation tube, any organism producing gas being considered 

 undesirable. Again liquefying organisms, as shown by the presence 

 of such bacteria in the gelatin plates, is evidence of probable contami- 

 nation by fecal bacteria. A question which seems difficult to decide 

 is as to the general nature of the so-called normal lactic acid bacteria 

 of milk. Some describe them as very short, broad bacilli with very 

 small colonies, fermenting lactose with the formation of lactic acid. 

 Others consider that the streptococci are the organisms which are 

 concerned with the normal fermentative changes. In examining 

 specimens of milk considered the best on the market, I have repeatedly 

 found the small red colonies on lactose litmus agar to be streptococci. 

 In connection with the organisms present in the tablets used for treat- 

 ing milk to produce lactic acid for the treatment of intestinal disorders, 

 and considered to be normal lactic acid bacteria, I have found both 

 streptococci and bacilli. These have all agreed, however, in not pro- 

 ducing gas in either lactose or glucose fermentation tubes. 



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 con- 

 tained in i c.c. of the milk. 



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 cen- 

 trifuge 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 haemacytometer pre- 

 paration as for blood and find the average number 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 



