182 



THE PLANT WORLD 



heating it to the boiling point (212° Fahrenheit ) for half an hour 

 or more lathis is one method of sterilization), and finally to protect 

 the culture -medium, as this food is called, from other living things 

 that, might feed upon it. 



Many bacteria thrive in a mixture of beef extract and gelatin 

 or agar-agar, a substance derived from seaweed. This mixture, 

 after sterilization, is poured into flat sterilized dishes with loose- 

 fitting covers. These Petri dishes, so called after their inventor, 

 are the traps in which we collect and study bacteria. 



Demonstration 2. Making a pure culture of bacteria. Transfer from 

 an infected and incubated culture medium some bacteria on point of a 

 sterile needle to the sterile surface of a Petri dish which contains sterile 

 agar. Watch the growth of the colonies for several days. Are these col- 

 onies all alike in appearance? 



How we may isolate bacteria of one kind from the other. In 



order to get bacteria of a given kind to study, it becomes necessary 

 to grow them in what is known as a pure culture. This is done 



after first growing the bacteria in 

 some medium such as beef broth 

 or gelatin, or on potato. When 

 the colonies of bacteria appear or 

 the beef broth becomes cloudy, 

 one form may be isolated from 

 the others by the following proc- 

 ess. A platinum needle is first 

 passed through a flame to steri- 

 lize it. After the needle is cooled 

 it is dipped in a colony contain- 

 ing the kind of bacteria we wish 

 to study. The needle is then 

 quickly drawn across the surface 

 of a dish of sterile culture medium, 

 and the dish is immediately 

 covered to prevent any other 

 forms of bacteria from entering. When we have succeeded in 

 growing only one kind of bacteria in a given dish, we have a pure 

 culture. 



Each spot on this culture medium indi- 

 cates a colony of bacteria. The different 

 sizes and shapes of the spots show that 

 there might be more than one type of bac- 

 teria present. How could you make a 

 pure culture, that is, one containing only 

 one kind of bacteria, if you have a steri- 

 lized culture medium and this Petri dish 

 containing colonies of bacteria ? 



