EXERCISE 76 

 TO STUDY THE GROWTH AND DISTRIBUTION OF BACTERIA 



Materials. Prepare a growth medium for bacteria by one of the 

 following methods. The first method is preferable, but the mate- 

 rials may not be available. 



Directions for work. 1. Secure as many petri dishes as will 

 be needed for the solution of the problems which are chosen 

 from the list on pages 96~97. Sterilize the dishes by heating in 

 a steam sterilizer for twenty minutes on each of three days, or by 

 heating one hour in a dry-air sterilizer to a temperature suf- 

 ficient to slightly scorch cotton (150 C.). The latter method 

 is preferable. Secure from the nearest bacteriological laboratory 

 as many tubes of sterile nutrient agar as you have petri dishes. 

 Heat some of the tubes in boiling water until the agar is melted, 

 when the temperature may fall to about 45 C., which will be 

 sufficient to keep the agar in liquid condition as long as neces- 

 sary. Pour the contents of each tube into a petri dish, cover, 

 and set aside to cool. Reserve the remainder of the tubes for 

 use as directed below. Using gummed labels, number each dish 

 and tube used. 



Great care must be taken in the above operations not to 

 allow bacteria to enter the agar either from the air or from any 

 other source. The petri dishes should be covered before the 

 last sterilization, and the covers should not be removed except 

 as directed in performing the experiments. The agar may be 

 poured into the dishes by lifting the cover on one side and 

 inserting the end of the test tube. 



2. In case the agar tubes mentioned above cannot be secured, 

 slices of potato may be substituted in the solution of certain 

 problems. Boil a potato the diameter of which is not greater 

 than that of a petri dish, boiling for ten or fifteen minutes 

 to partly sterilize the surface. Cut into thin slices and . place 

 one in each petri dish. Sterilize in steam as directed above. 

 Sterilization should be repeated once or twice. 



3. Certain of the problems suggested below may be solved 

 by the use of sterilized milk as a medium for the growth of 



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