PROBLEM I . How We Are Protected Agamst Microorgau'mns 3 1 7 



5. What is the advantage of a solid culture medium? By whom was this 

 method devised? How is a pure culture made? 



6. In what different ways are bacteria identified? In order to identify 

 them microscopically what is necessary? 



7. What is a bacterial spore? Describe the effect of extreme heat, ex- 

 treme cold, dryness, and chemicals on bacteria. Make exact, not gen- 

 eral, statements in your answer. Distinguish between antiseptics and 

 disinfectants. 



8. By what method do bacteria enter the body? By what routes do they 

 enter? 



9. What is the body's first line of defense? What is the second line? 

 Describe the structure and work of phagocytes. Where would you 

 look for cells that engulf bacteria? 



10. Define toxin and antitoxin. What is meant by saying an antitoxin is 

 specific? Name and describe the action of three other substances be- 

 sides antitoxin which act against bacteria or their products. 



1 1 . Explain how the body acquires immunity. Why are phagocytes not 

 listed among the antibodies? Name some antibodies and tell what each 

 does. Discuss the length of time that acquired immunity may last. 

 How has this information been made use of for many centuries? What 

 other kind of immunity is there besides acquired immunity? Explain. 



Exercises 



1. How may bacteria be raised in a liquid culture medium? Heat to- 

 gether in a saucepan 250 cc (a glass) of water, 2 2 grams of salt, 1^ grams 

 of peptone and 2^ grams of Liebig's beef extract. Filter into test tubes, 

 filling them about i full. Close each test tube by stuffing a wad of cotton 

 into the mouth. Stand the test tubes in boiling water for a minute. Put a 

 very small amount of material scraped from the inside of your mouth 

 into each of several tubes. Close them again. Place the tubes in a warm 

 place for several days. What do you notice? What may account for the 

 change in the broth? How could you find out? 



2. How may nutrient agar be prepared? Heat together the ingredients 

 mentioned in Exercise i (using four times as much of each) and add 10 

 grams of agar. Put in baking soda until red litmus paper is turned blue by 

 the mixture. When the agar is dissolved, filter the mixture through thick 

 absorbent cotton lining a funnel. Warm the cotton and funnel before 

 they are used. Why? Allow the filtered substance to flow into a glass 

 flask. Plug the mouth of the flask with a wad of sterile absorbent cotton. 

 Why is the plug used? Why is a cotton plug used instead of a rubber 

 stopper? If you wish to use this nutrient agar for studying special kinds 

 of bacteria or if you wish to keep it, what must you do to the agar in 

 the flask? Why? If you wish to pour the agar into Petri dishes, what 

 should you do to the Petri dishes in advance? Use 10 to 15 dishes. When 

 you are ready to transfer the sterile agar into sterile Petri dishes, you 



