258 BACTERIA IN MILK AND THEIR SOURCES 



Inoculate three more plates in a similar manner, and incubate 

 all at 1 8 to 20 ; count the colonies appearing in seventy-two 

 hours, and calculate the average number on each. 



The counting can be facilitated by using a Pake's disc (Fig. 45) 

 placed beneath the plates. 



From the average number of colonies on a plate, the dilution 



2 



ff 



01 6 



FIG. 45. Pake's counting disc. 



of the milk, and the amount of the latter used for inoculation, 

 the number of the organisms in i c.c. of the original sample can 

 be calculated thus : 



No. on plate x dilution x amount used f ^ r inoculation - no. of organisms 

 in the original sample. 



E,g. If 420 colonies are found on a plate inoculated with 

 .5 c.c. of the milk diluted 1000 times, the number present in 

 i c.c. of the original sample is 420 x 1000 x -J = 840,000. 



Ex. 126. Repeat the above but use a lactose agar medium 

 instead of gelatine, and incubate at 37 C. for forty-eight hours. 



Ex. 127. Milk 50 c.c. of "foremilk," "midmilk," and "strip- 

 pings " from the same quarter of the cow's udder into three sterile 



