256 CLEAN MILK 



ing for record the count obtained on that plate which yields between? 

 40 and 400 colonies. 



Plating undiluted 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 conditions, 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 recom- 

 mendations of the Committee on Water Analysis, except that the 

 percentage of agar shall be 1% and the reaction -\- 1.5. 



All variations from agar media made as described shall be con- 

 sidered 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. 



Storage of Media. — Media may be made up in quantity,, 

 tubed and stored (preferably in an ice chamber). 



PLATING.f 



Plating Apparatus. — For plating it is best to have a single 

 water bath in which to melt the media and a water- jacketed water 

 bath for keeping it at the proper temperature ; a wire rack, which 

 should fit both of the water baths, for holding the media tubes;. 



* The old practice was to plate, not 1 c.c. of a dilution, varying the dilution to suit 

 conditions, as described above, but to plate the undiluted sample in fractional portions of 

 ace. While mathematically the same amount of milk is plated, whether -^ c.c. of 

 undiluted milk or i c.c. of a dilution of i to 99 be used, in practice the latter is superior, 

 for the reasons given above. 



f A method of avoiding troubles due to moisture consists of inverting the plates and 

 putting in the lid of each petri dish a strip of blotting paper on which there is a large 

 drop of glycerine. Incubate as directed. (Extract from 1908 Report of American . 

 Public Health Association.) 



