BACTERIOLOGICAL EXAMINATIONS 349 



Bacteriological Examinations 



Dr. Slack writes as follows : 



" We centrifugalize the milk in small glass tubes (about 2 c.c. each, 

 the ends being closed with rubber stoppers). Our apparatus carries 

 20 tubes and we centrifugalize for ten minutes at a speed of 2000- 

 3000 revolutions a minute. 



" The sediment obtained on the rubber stopper is smeared evenly 

 with a drop of sterile water over a space 4 sq. cm. By examining this 

 sediment with a 1-12 oil immersion lens, we determine the presence 

 of pus or streptococci and are also able to make a microscopic estimate 

 of the number of bacteria present." 



Since it is impossible to differentiate between dead leucocytes and 

 pus, and since a certain number of leucocytes are normal (3 or 4 in 

 a 1-12 immersion lens field) in milk, it is necessary to fix an arbitrary 

 standard not to be exceeded by these cells. The standard, observed 

 by the Boston Board of Health, is 50 cells to the field of a 1-12 oil 

 immersion lens (spreading the sediment from 2 c.c. of milk over a 

 surface of 4 sq. cm.). If this number is exceeded the milk is con- 

 demned. After the milk is centrifuged in small glass tubes (see above), 

 the sediment is placed on spaced, glass slides, dried with gentle heat 

 and stained with methylene blue. During the course of the microscopic 

 examination for pus, the number of bacteria can be determined with 

 a very fair degree of accuracy without plating the milk if the milk 

 contains more than 1 00,000 bacteria in 1 c.c. 



Milk is condemned by the Boston Board of Health for streptococci 

 when. 3 tests are positive: 1. When the centrifuged sediment shows. 

 streptococci, cocci or diplococci. 2. When the plate from the same 

 sample shows colonies resembling streptococci colonies, in excess of 

 100,000 to 1 c.c. 3. When such colonies transferred to broth and 

 grown for 24 hours at 37 C. show streptococci alone or in great ex- 

 cess of the other bacteria present. 



" The examination of milk for pus was first suggested by Dr. 

 Stokes of Baltimore, and has since been carried out in a number of 

 public health laboratories in different parts of the United States. The 

 researches of Stokes, Bergey, Stewart^ Doane, Slack and others have 



