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Exercise 9 Isolation of Pure Cultures of Bacteria 
MATERIALS: 
3 tubes of plain agar for slopes 
3 tubes of plain broth 
1. Pure cultures are to be made from the colonies described in Exercise 8. 
A pure culture of a bacterium, yeast or mold is a culture which contains only 
one kind of a bacterium, yeast or mold. It is assumed that each of the three 
colonies selected have originated from a single organism. 
2. Mark three tubes of agar, 1, 2, 3 respectively. If the agar is not sloped 
in the tubes, melt and slope it as shown by the instructor. 
FIG. 13—“FISHING” OFF COLONIES FROM A PETRI DISH CULTURE 
3. When the agar has become firm, inoculate tube No. 1 from colony No. 
1 (Exercise 8) as follows: Heat the striaght wire needle in the flame to red¬ 
ness to sterilize it. When cool, touch the tip of the needle to the colony 
