146 BACTERIOLOGICAL EXAMINATION OF MILK. 



Me;dia. 



The standard medium for determining the number of bacteria in milk 

 shall for the present be agar, made according to the recommendations of 

 the Committee on Water Analysis (1), except that the percentage of 

 agar shall be 1 per cent, and the reaction +1.5 (2d). 



All variations from agar media made as described shall be considered 

 as special media. 



Much work 3'et remains to be done on media ; the above is recom- 

 mended as giving the highest and most uniform counts as far as our com- 

 parative work has extended and with but slight variations is the medium 

 in most common use. 



Storage of media. Media may be made up in quantit3', tubed and 

 stored (preferably in an ice chamber). 



Pirating. 



Plating apparatus (11). For plating it is be,st 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 ; a thermometer for 

 recording the temperature of the water in the water- jacketed bath ; ster- 

 ile 1 cc. pipettes ; sterile petri dishes ; and sterile dilution water in 

 measured quantities. 



For milk work porous earthenware petri dish covers (16), are much 

 superior to glass covers, since they absorb the excess moisture from the 

 agar and prevent "spreading." 



It is quite essential to the best results that the porous covers should 

 be wet as seldom as possible. In sterilizing them the process should be 

 prolonged over the time necessary to kill the organisms in order that 

 the covers maj- be thoroughl}- dry. 



Extract from the 190S report. A method of avoiding the 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. 



As a result of these experiments we feel that the agar plates with 

 glycerine, prepared in the manner above indicated, are slightly more 

 reliable than the earthenware tops. 



The principal objections to the latter are : 



(1) They scratch the glassware. 



(2) The plate has to be uncovered for examination, and 



(3) They are more liable to dry out if kept" longer than 36 hours. 

 Straight-sided 1 cc. pipettes are more easily handled than those with 



bulbs ; they may be made from ordinary glass tubing about -^^ of an inch 

 in diameter and calibrated in the laboratory (17). They .should be made 

 about ten inches in length. 



Digitized by Microsoft® 



