PROBLEM 239 



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sample according to the following directions: With a sterile 

 bulb pipette draw off 1 c.c. of milk from a well-shaken sample 

 bottle. Add to this 19 c.c. of distilled water, taking care to have 

 the water in a sterile test tube, protected from any dust by an 

 absorbent cotton plug. After mixing the contents of the tube 

 thoroughly, quickly flood the surface of a sterile Petri dish con- 

 taining agar culture media with the mixture of milk and water. 

 Drain the dish, keeping it covered during the operation ; label ; 

 fasten down the cover with strips of paper; and place to one 

 side. Treat each of the other samples of milk in the same 

 manner as just described, taking care to label each as to the 

 source of the milk, etc. Place the dishes side by side in a mod- 

 erately warm place. 



Observations. After two days, and on each successive day for 

 a week, examine the different Petri dishes. Count the number of 

 colonies in each dish. 

 Also note the different 

 kinds of colonies of 

 bacteria present in 

 each of the Petri 

 dishes. Tabulate the 

 results. 



Conclusion. 1. 

 Which of the grades of milk examined seems to be most free from 

 bacteria? 



2. Should milk be entirely free from bacteria? What do the 

 bacteria present in greatest quantities probably do to the milk? 



3. If several kinds of bacteria are present in milk, what can you 

 say of its purity? What ought to be done with such milk before 

 it is used? 



Problem 239 : To determine the bacterial content of some 

 kinds of water. 



Materials. See Problem 238. Omit milk and substitute 

 samples of water. 



Method. By means of a sterile bulb pipette place, in sterile 

 Petri dishes containing agar culture media, equal amounts of 



