3i8 Constant Care Is Needed for Health unit vi 



must heat the flask to Hquefy the agar mixture and then pour it in such 

 a way that as few bacteria as possible will have a chance to enter during 

 the pouring. What precautions would you suggest? Consult with your 

 teacher to make sure that you have thought of all possible precautions. 

 Some of the medium may be poured into sterile test tubes laid on a table 

 at an angle of about 30°. 



3. What can be learned about bacteria grown on agar plates? Pour a 

 very little (10-15 drops) broth containing bacteria into a sterile agar 

 plate and tilt to spread the liquid well over the surface. Close the Petri 

 dish and set it aside in a warm place (about 80° F). Examine every morn- 

 ing and evening. How long does it take before you see bacteria on the 

 agar? Why can the bacteria now be seen? Each colony contains millions 

 of bacteria. What does this indicate about their rate of reproduction? 

 What are some of the differences among the colonies? Of what use might 

 a knowledge of these differences be to bacteriologists? 



4. Deinonstration. To show how to make a pure culture of bacteria. 

 Pure cultures are grown best on agar slants. Pour some sterile agar into 

 a sterile test tube and plug it with a wad of absorbent cotton. Then place 

 the test tube in a tilted position at an angle of about 30 degrees until the 

 agar hardens. In one of your exposed Petri dishes you will no doubt find 

 a colony of a striking color. Transfer bacteria from this colony. Before 

 transferring the bacteria to the slant, what must you be careful to do with 

 the needle you employ? Would you recommend heat or disinfectants? 

 Why? Holding the needle in the flame until it is red hot and then allow- 

 ing it to cool for a moment before applying it to the colony of bacteria 

 will prove successful. Why is a test tube better than a Petri dish for mak- 

 ing a pure culture? 



5. Dejii07istration. To show how staining helps in the study of bacteria. 

 On each of two slides smear a small drop of the broth containing bacteria. 

 Permit the slides to dry. Then pass them through a flame rapidh' three 

 times with the side that contains the bacteria uppermost. Cover the ma- 

 terial on one slide with some dye like methylene blue or gentian violet 

 and allow it to stand for two minutes. Wash the dye off with water and 

 examine the slide under the oil immersion lens of the microscope. Com- 

 pare the unstained slide with the stained one. What do you note? 



6. Where are bacteria found? Expose a number of sterile Petri dishes 

 containing sterile agar to the air in your classroom, in the school corridor 

 and playgrcnind, and in other places for several minutes. In other dishes, 

 place various objects, such as chalk dust, floor dust, food, and saliva. 

 Label each of the dishes and put them away in a warm place for a few 

 days. What do you find? Can you be sure that the bacteria came from the 

 various objects or places to which the Petri dishes had been exposed? 



7. Devise and perform an experiment to find out the relative number 

 of bacteria in your classroom and in a room in your home. What will 

 you need for your experiment? 



