NUMBER AND SOURCE OF BACTERIA IN MILK 259 



flasks. Keep them cool, and after arrival at the laboratory 

 determine as soon as possible the number of organisms in i c.c. 

 of each by the method described in Ex. 125, using both 

 lactose gelatine and agar media. 



Ex. 128. Take four sterile Petri dishes and pour into each 

 the contents of a tube of lactose gelatine medium. Cover and 

 allow the medium to solidify. 



Prepare a similar number of lactose agar plates. 



Take them to the farm ; remove the covers and expose the 

 surface of one of the gelatine and one of the agar plates for one 

 minute : 



(1) In the open air. 



(2) In the cowshed before the cattle are brought in for 



milking. 



(3) Under or near the udder of the cow during milking. 



(4) In the cowshed after it has been swept out. 



After the stipulated time of exposure of the plates put on the 

 covers, wrap them in paper and take them to the laboratory. 



(a) Incubate the gelatine plates at 20 C. 



(b) Incubate the agar plates at 35 C. 



Count and compare the number of colonies appearing on the 

 two media in 12, 24, 48, and 72 hours, noting the number which 

 liquefy the gelatine. 



Ex. 129. Note the number of colonies on the plate which 

 has been exposed for thirty seconds under or near the udder 

 during milking, as in the preceding experiment. Calculate the 

 area of the exposed surface of the plate by multiplying the square 

 of half its diameter by 3. 14. Find the area of the opening of the 

 milking pail in the same way, and from this and the number 

 of bacteria found to fall on the estimated surface of the plate 

 in one minute calculate the number which fall into the pail 

 during the whole time of milking. 



Ex. 130, Prepare Petri dishes of lactose gelatine and agar 

 media. 



