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Chemjjc Chemjjc
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8 years ago
1.   Why is it necessary to stain bacteria in order to see them?
2.   What is a smear and how do you make one?
3.   What is the purpose of “fixing” a slide that is to be stained? How do you do it? What do you think would happen if you forgot this step?
4.   List and define the 3 basic shapes of bacteria. What are the dimensions of an average bacillus (in micrometers)? A coccus?
5.   What is the function of the iodine solution in the Gram stain? If it were omitted, would the staining results be affected? Why or why not?
6.   What is the purpose of the alcohol solution in the Gram stain?
7.   What counterstain is used in the Gram stain? Why is it necessary?
8.   Describe at least 3 conditions in which an organism might stain “Gram-variable” (aka… What can go wrong and how will it affect the outcome?).
9.   Describe how to perform a capsule stain? Which reagents/dyes/etc. are used and what do they do?

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Valued Member
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8 years ago
Yikes, this is more than the maximum allotted two questions per thread! Hence I will only answer the first two - feel free to start a new thread for the rest.

1.   Why is it necessary to stain bacteria in order to see them?

I think the question answers itself - to see them better in a light microscope! You stain specimens to show up different structures, such as their cell wall - it is the peptidoglycan that holds the stain I believe. Gram negative bacteria usually have an extra outside layer on the cell wall.

2.   What is a smear and how do you make one?

Remember, to stain bacteria, thin film of bacterial cells must be placed on a slide, this is a smear. Spreading a small amount of a bacterial broth culture on a clean slide and allowing it to dry. From solid medium: place one or two loopfuls of water on the slide, transfer a very small amount of the culture with a sterile loop and mix with the water on the slide, allow the smears to air dry at room temperature, and pass the slide through the flame of a burner two or three times. From liquid medium: place two or three loopfuls of the liquid culture on the slide with a sterile loop, spread the bacteria within the circle, allow the smears to air dry at room temperature, and cover the smears with 95% methanol for one minute, and then let the smears air dry.
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