TBR Bio Chapter 6 Passage 11

[LakersInSix]

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#68 After being subjected to the Gram staining procedure, Gram-positive and Gram-negative cells show which of the following colors, respectively?

answer is D. Blue;red

The solution says alcohol takes out color from gram-negative because of its lower peptidoglycan content...are we expected to know that alcohol does this? (below is the word by word answer solution for what alcohol does)

"In Step 3 alcohol is added and acts to decolorize the cells. However, only the Gram-negative cells are decolorized by the alcohol. The Gram-positive cells remain purple. This is due to the thickness of the peptidoglycan layer of the Gram-positive cells. Alcohol tends to dehydrate this layer. thus making any pores within the layer itself rather small. These small pores hinder the passage of the crystal violetiodine
complex during the extraction process. Since the crystal violet-iodine complex remains trapped in the peptidoglycan layer, Gram-positive cells still display a violet color at this stage. In Gram-negative bacteria the thin peptidoglycan layer does not significantly hinder the extraction process, and these cells therefore display no color (i.e., they are colorless) at this stage."

 

[illinichief89]

All you need to know is that gram positive bacteria stain violet/blue while gram negative bacteria stain pink/red. And the reason for this is the difference in peptidoglycan thickness.

They are just explaining the mechanism to be thorough.

 

[LakersInSix]

All you need to know is that gram positive bacteria stain violet/blue while gram negative bacteria stain pink/red. And the reason for this is the difference in peptidoglycan thickness.

They are just explaining the mechanism to be thorough.

Ah ok thanks. The MCAT really wanting us to know the color is nuts! I just remember gram positive stains because of its thicker peptidoglycan content

So I guess I was supposed to realize violet/purple mixed with red become blue? Is there an easy way to see that?

 

[illinichief89]

In gram staining, you start by staining the bacteria with crystal violet dye. Next you stain with iodine. Now all bacteria will be stained a violet/blue color at this point. The 3rd step is the alcohol step, which is to decolorize any gram negative bacteria. After this step the bacteria will either appear clear or remain a violet/blue color, although they will be less noticeably colored. The final step in gram staining is staining with Safranin dye, which is pink in color. Remember some bacteria were clear after the alcohol step, right? Well after the safranin step they will be a Pink/red color because they absorb the dye. These are the gram negative bacteria. The gram positive bacteria will remain violet, even after this step. Its an important staining technique in microbiology to distinguish between these two groups of bacteria. This is what it looks like under a microscope:

And here are the steps in picture form:

 

[LakersInSix]

I don't think that's exactly right. The solution specifically says that adding the red stain turns the violet stain into blue. I just have no clue how red+purple = blue

here is what it says:
"In Step 4 a counterstain (safranin) is added to the suspension. This red-colored stain is added so the Gram-negative bacterial cells can be visualized. They now display a red color. The purple colored Gram-positive bacteria also pick up the red stain and now appear blue."

 

[brownbaglunch]

I don't think that's exactly right. The solution specifically says that adding the red stain turns the violet stain into blue. I just have no clue how red+purple = blue

here is what it says:
"In Step 4 a counterstain (safranin) is added to the suspension. This red-colored stain is added so the Gram-negative bacterial cells can be visualized. They now display a red color. The purple colored Gram-positive bacteria also pick up the red stain and now appear blue."

I remember being equally frustrated by this explanation. The above post about the mechanics of the gram stain (with picture) are right on. If you take the text explanation and swap blue/purple, you get blue + red = purple, which is correct.

When performing a gram stain, depending on the age of the colony, the amount of decolorizing and counterstaining you do, the G+ bacteria appear blue, indigo or violet, and the G- appear pink or red. In class we put two control groups on the same slide as the sample.

While TBR is excellent review material, some of their questions are hard in ways that are different than the way AAMC practice questions are hard. Don't let it get you down, keep at it. The time you spend contemplating things like this will help you, even if it makes you pull your hair out now =)