244 MILK HYGIENE 



Media may be made up in quantity, tubed and stored 

 (preferably in an ice chamber). 



Plating: 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 ; a ther- 

 mometer for recording the temperature of the water in 

 the water- jacketed bath; sterile 1 c.c. pipettes; sterile 

 petri dishes; and sterile dilution water in measured 

 quantities. 



For milk work porous earthenware petri dish 

 covers are much superior to glass covers, since they 

 absorb the excess moisture from the agar and prevent 

 66 spreading. " 



It is quite essential to the best results that the porous 

 covers should be wet as seldom as possible. In steril- 

 izing them the process should be prolonged over the 

 time necessary to kill the organisms in order that the 

 covers may be thoroughly dry. 



Straight-sided 1 c.c. pipettes are more easily handled 

 than those with bulbs ; they may be made from ordinary 

 glass tubing about 3 / 16 of an inch in diameter and cali- 

 brated in the laboratory. They should be made about 

 10 inches in length. 



Plating technique. The agar after melting should 

 be kept in the water- jacketed water bath between 40 C. 

 and 45 C. for at least fifteen minutes before using, to 

 make sure that the agar itself has reached the tempera- 

 ture of the surrounding water. If used too warm the 

 heat may destroy some of the bacteria or retard their 

 growth. 



For routine work in cities in order to bring down the 

 actual number of colonies in a plate around the standard 

 of two hundred, it is well to use a dilution of 1-10,000. 

 To make this dilution use two bottles of sterile water 

 each containing 99 c.c. 



Shake the milk sample twenty-five times, then with a 

 sterile pipette remove 1 c.c., put into the first dilution 

 water and rinse the pipette by drawing dilution water 

 to the mark and expelling; this gives a dilution of 1 

 to 100. 



