Exercise 1: Hanging-drop and wet-mount preparations, Exercise 2: Simple stains, Exercise 3: Gram stain, Exercise 4: Pure bacterial colonies, Exercise 5:

Cassandra Howery

Week 5 Assignment 2

Microbiology Laboratory

Week 5 Review Sheet I

Exercise 1: Hanging-drop and wet-mount preparations

1.   How does true motility differ from Brownian movement?

2.   What morphological structure is responsible for bacterial motility?

3.   Why is a wet preparation discarded in disinfectant solution or biohazard container?

4.   What is the value of a hanging-drop preparation?

5.                       What is the value of a wet-mount preparation?

Exercise 2: Simple stains

1.   Define acidic and basic dyes. What is the purpose of each?

2.   What is the purpose of fixing a slide that is to be stained?

3.   Why are the specimens to be stained suspended in sterile saline or distilled water?

4.   How does a stained preparation compare with a hanging drop for studying the morphology and motility of bacteria?

5.   List at least three types of bacteria whose names reflect their shapes and arrangements, and state the meaning of each name.

Exercise 3: Gram stain

1.   What is the function of the iodine solution in the Gram stain? If it were omitted, how would staining results be affected?

2.   What is the purpose of the alcohol solution in the Gram stain?

3.   What counterstain is used? Why is it necessary? Could colors other than red be used?

4.   What is the advantage of the Gram stain over a simple stain such as methylene blue?

5.   In what kind of clinical situation would a direct smear report from the laboratory be of urgent importance?

Exercise 4: Pure bacterial colonies

1.   When an agar plate is inoculated, why is the loop sterilized after the initial inoculum is put on?

2.   Distinguish between a pure culture and a mixed culture.

3.   Define a bacterial colony. List four characteristics by which bacterial colonies may be distinguished.

4.   Why should a Petri dish not be left open for any extended period?

5.                       Why does the streaking method you used to inoculate your plates result in isolated colonies?

Exercise 5: Pour plate and streaking technique to obtain pure cultures

1.   Discuss the relative convenience of pour- and streak-plate techniques in culturing clinical specimens.

2.   How do you decide which colonies should be picked from a plate culture of a mixed flora?

3.   Why is it necessary to make pure subcultures of organisms grown from clinical specimens?

4.   What kinds of clinical specimens may yield a mixed flora in bacterial cultures?

5.                      When more than one colony type appears in pure culture, what are the most likely sources of extraneous contamination?

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