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Hanging-Drop and Wet-Mount Preparations; Simple and Gram Stains; and Pure Bacterial Colonies and Cultures

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 Bram 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 of time?
5. Why does the streaking method used to inoculate 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?

Solution Preview

Exercise 1: Hanging-drop and wet-mount preparations

1. How does true motility differ from Brownian movement?

The difference between true motility and Brownian movement are that true motility is when the movement of cells is caused by an intentional force or appendage. The movement of cells is more purposeful and in a specific direction. On the other hand, Brownian motion is when cells are bombarded by water molecules, which caused them to appear as to be vibrating. The Brownian movement is random and not in any specific direction. (1)

2. What morphological structure is responsible for bacterial motility?

The morphological structure that is responsible for bacterial motility is the flagella.
Flagella are semi-rigid structures that is used to move microbial cells; they are attached to the cell through a complex of proteins called the hook and basal body. The flagella can rotate and cause the cell to move by the proton motive force. (2) Flagella are responsible for motility in liquid.

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

A wet preparation is an application of a small amount of bacterial suspension to the slide, and is useful for giving clear images of fresh specimens under the microscope. A sample of liquid culture is place on the slide. It is discarded in disinfectant solution in order to kill the bacterial or pathogen suspension. It is disposed in biohazard container.

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

The value of the hanging drop procedure over the Wet Mount is that the hanging drop procedure enables the organism to live longer for a longer period. The hanging drop procedure enables the organism to not dry out rapidly, and it provide a detailed analysis of the motility of any organism. (3)

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

The value of the wet-mount preparation is that it can show the motility in the microorganisms.

Exercise 2: Simple stains

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

Dyes are acidic or basic depending on the charge they carry. An acidic dye carries a negative charge after loss of a hydrogen while a basic dye carries a positive charge due to a gain of hydrogen or loss of hydroxide. (4 )

The purpose of an acidic dye is to stain basic tissue proteins during microscopic examinations. (4)

The purpose of a basic dye is to stain cell nucleus and acidic components of tissues. (4)

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

The purpose of fixing a slide ...

Solution Summary

The expert examines hanging-drop and wet-mount preparations. The morphological structures which are responsible for bacterial motility are given.

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