BACTERIAL MILK ANALYSIS 243 



The 1-100 dilution is prepared in the same way, ex- 

 cept that a bottle with 99 c.c. of sterile water is substi- 

 tuted for the test tube. 



The 1-1,000 dilution is prepared by first making the 

 1-100 dilution, shaking twenty-five times and trans- 

 ferring 1 c.c. of the dilution to a test tube containing 

 9 c.c. of sterile water. 



The 1-10,000, 1-100,000 and 1-1,000,000 dilutions are 

 made in the same manner by dilutions of the 1-100, 

 1-1,000 and 1-10,000 dilutions, 1 c.c. to 99 c.c. of sterile 

 water. 



It is recommended that that dilution be used which 

 will produce about two hundred colonies to a plate, 

 ranging from 40 to 400; where a 1-10 dilution exceeds 

 this number the 1-100 dilution is more accurate, etc. 

 The number of bacteria present may if desired be ap- 

 proximately estimated before dilutions are made by 

 direct microscopic examination of a properly prepared 

 sediment. Otherwise it is necessary to make a range of 

 dilutions, thereafter selecting for record the count ob- 

 tained on that plate which yields between 40 and 400 

 colonies. 



Plating whole milk is unreliable, whatever quantities 

 be used, since the bacteria are not so well separated as 

 in the dilutions, and often, owing to the crowded con- 

 ditions, only a portion of the bacteria present will 

 develop into visible colonies. Moreover, if a cubic 

 centimeter of the milk is used, the turbidity of the jelly, 

 due to the presence of the milk, hides the colonies 

 present from the eye. 



Media. The standard medium for determining the 

 number of bacteria in milk shall for the present be agar, 

 made according to the recommendations of the Com- 

 mittee on Water Analysis, except that the percentage of 

 agar shall be 1 per cent, and the reaction + 1.5. 



All variations from agar media made as described 

 shall be considered as special media. 



Much work yet remains to be done on media; the 

 above is recommended as giving the highest and most 

 uniform counts so far as our comparative work has 

 extended and with but slight variations is the medium 

 in most common use. 



