296 CLEAN MILK 



actly one-tenth of a cubic millimeter. Fill this counter with the 

 stained sediment, cover with a cover-glass, allow to stand for about 

 one minute for the leucocytes to settle, and then, placing under a 

 microscope, count the number of leucocytes found in a single ruled 

 square and calculate the numbers in the whole chamber. Remem- 

 bering then that the chamber contains one-tenth of a cubic milli- 

 meter of the stained sediment, it is easy to calculate the number of 

 leucocytes in the original milk. This should be calculated upon the 

 basis of a certain number per c.c, and good milk should not have 

 more than 500,000 per c.c. * 



5. Separation of the Common Species of Bacteria from 

 Milk. Allow some milk to stand at about 70 . As soon as it be- 

 gins to sour, but before it curdles, dilute 1 c.c. 500,000 times and 

 make litmus gelatin plates. Allow to grow four days at 70 ° unless 

 liquefiers make it impossible. 



a. Determine the total number of bacteria in 1 c.c. of milk. 



b. Determine the number of acid bacteria per c.c. 



c. Determine the number of liquefiers per c.c. 



d. Determine the percentages in each case. 



e. Isolate a colony of Bad. lactis acidi. This may be recog- 

 nized as follows : It is an intensely acid colony, rather opaque, 

 always below and never on the surface. It is small, only just 

 visible to the naked eye. and when examined under a low power 

 microscope it frequently, though not always, shows a slight rough- 

 ness, looking like minute spines around its edge. With a platinum 

 needle lift out one of these colonies and inoculate into a gelatin 

 tube by stabbing the needle directly into the gelatin. After growth, 

 set aside for future use.f 



* In healthy cows the leucocytes in milk may range as high as i,Soo,ooo (Rus- 

 sell) to 4,500,000 (Savage). In healthy cows the leucocytes average over 100,000 

 (Savage), or 2000,000 (Doane). 83$ of cows with mastitis (Russell) yield milk con- 

 taining over 500,000 leucocytes per c.c. Prolonged retention of milk in the udder 

 gives rise to increased leucocyte content. — Winslow. 



t For special characteristics, see Conn's " Practical Dairy Bacteriology." 



