Download Staining for Differences

Name ________________________________ Class _______ Date _________________
Staining for Differences
Use the Microscopy lab bench to learn how
microbiologists use stain microscope slides to
tell different types of bacteria apart.
Lab Bench Used
Under a powerful light microscope, it’s possible to see
something as small as a single bacterium. However, different types of bacteria can
look very similar even at high magnification. In such cases, scientists use a variety of
stains to tell types of bacteria apart. This technique is called differential staining. In
this activity, you will observe how different types of bacteria appear when stained
using this technique.
Enter the Virtual Bio Lab and select the title of this lab activity from the “Organisms
& Natural History” menu on the whiteboard. You will be taken to the virtual
Microscopy lab bench.
Click on “Human” in the Species Selector. Look through the compound microscope
menu for images of various bacteria associated with humans.
1. What bacterial shapes are visible?
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Many species of bacteria have the basic shapes you observed in Step 1. One method
scientists can use to tell similar-looking bacteria apart is called the Gram stain.
Depending on what their outermost coverings (i.e. cell walls, membranes or other
coverings) are made of, Gram-stained bacteria turn either pink or purple. Grampositive bacteria turn purple, while Gram-negative bacteria turn pink. Roll over the
thumbnails in the compound microscope image menu. Click on the image titled
“Cholera bacteria (Gram stain).” Next, choose “Strep bacteria (cocci) (Gram stain).”
2. Which bacteria are Gram-positive, and which are Gram-negative?
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The Gram stain doesn’t work on all species of bacteria. The acid-fast bacteria, for
instance, have a unique, acid-resistant lipid cell wall that interferes with the Gram
staining process. However, the acid-fast stain combines with this lipid covering.
Acid-fast organisms appear red against a blue background in an acid-fast stain.
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Virtual Bio Lab
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Organisms & Natural History:
Staining for Differences
Name ________________________________ Class _______ Date _________________
3. Click on the thumbnail titled “Tuberculosis bacteria (acid-fast stain)”. Record
your observations below.
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Some bacteria form hardy structures known as endospores. An endospore is a
dormant cell that is highly resistant to heat (including boiling) drying out, nutrient
depletion, and physical damage.
4. Click on the images titled “Botulism bacteria (malachite stain)” and “Botulism
bacteria (Gram stain).” The endospores will be visible as large lumps at the ends
of some of the bacteria. Record your observations below.
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Some bacteria have flagella—whip-like structures used for locomotion. Often, they
are difficult to see under the microscope because they are very thin. A technique
called the flagella stain coats bacterial flagella in stain, making them thicker and
more visible.
5. Click on the thumbnail titled “Flagellar bacteria.” Record your observations.
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6. Suppose you have a sample consisting of a mixture of rod-shaped and spherical
bacteria. Using only the Gram stain, what is the greatest number of types of
bacteria could you possibly find in this sample? Explain your reasoning.
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Virtual Bio Lab
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Organisms & Natural History:
Staining for Differences
Name ________________________________ Class _______ Date _________________
7. You are trying to identify a bacterial culture that contains of only one species of
rod-shaped bacteria. You perform a Gram stain three times. Each time a few
individual cells appear pink, some look purple, and others appear unstained.
Provide one reason why the Gram stain in this case might be inconclusive. What
might be your next step in identifying these bacteria? Explain your reasoning.
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8. Staining isn’t the only way scientists tell different types of bacteria apart. Suggest
another method that you think would work to distinguish different types of
bacteria that appear similar under the microscope. Explain how this method would
work. Hint: Different species may have different nutritional or metabolic
requirements.
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Virtual Bio Lab
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Organisms & Natural History:
Staining for Differences