Q: Which equation best represents the formation of the crystal violet-iodine complex in Gram-positive cells?

$$\mathrm{Crystal\ violet} + \mathrm{Iodine} \rightarrow \mathrm{Crystal\ violet\text{-}Iodine\ complex}$$

Question 1

Which component of the bacterial cell wall is primarily responsible for the differential outcome of the Gram stain?

Accepted Answer

Peptidoglycan

Question 2

During the Gram stain procedure, what is the role of Gram's iodine?

Accepted Answer

It serves as a mordant to form a complex with crystal violet.

Question 3

Why do Gram-negative bacteria appear pink after the Gram stain procedure?

Accepted Answer

They are decolorized and then take up the safranin counterstain.

Question 4

Which step in the Gram stain procedure is most affected by the age of the bacterial culture?

Accepted Answer

Decolorization

Question 5

A microbiologist performs a Gram stain but leaves the decolorizer on for too long. What is the most likely result?

Accepted Answer

Gram-positive cells may appear pink, similar to Gram-negative cells.

Question 6

Which genus requires the use of an acid-fast stain due to the presence of mycolic acid in its cell wall?

Accepted Answer

Mycobacterium

Question 7

What is the primary stain used in the acid-fast staining procedure?

Accepted Answer

Carbolfuchsin

Question 8

After performing an acid-fast stain, how would acid-fast cells and non-acid-fast cells appear under the microscope?

Accepted Answer

Acid-fast cells appear red or pink, non-acid-fast cells appear blue.

Question 9

Which chemical is used as the decolorizer in the acid-fast staining procedure?

Accepted Answer

Acid alcohol

Question 10

Why is heat applied during the acid-fast staining procedure?

Accepted Answer

To allow carbolfuchsin to penetrate the mycolic acid layer

Question 11

Which of the following best explains why Gram-negative bacteria are more susceptible to decolorization than Gram-positive bacteria?

Accepted Answer

Gram-negative bacteria have a thinner peptidoglycan layer.

Question 12

A sample contains both Staphylococcus aureus and Mycobacterium smegmatis. After an acid-fast stain, which color would you expect each organism to appear?

Accepted Answer

Staphylococcus aureus will appear blue, Mycobacterium smegmatis will appear red.

Question 13

Which of the following is NOT a critical factor to consider when performing a Gram stain?

Accepted Answer

Type of microscope used

Question 14

If a Gram stain is performed on a mixed culture and all cells appear pink, which of the following is the most likely explanation?

Accepted Answer

The decolorizer was left on too long.

Question 15

Which equation best represents the formation of the crystal violet-iodine complex in Gram-positive cells?

Accepted Answer

$$\mathrm{Crystal\ violet} + \mathrm{Iodine} ightarrow \mathrm{Crystal\ violet	ext{-}Iodine\ complex}$$

Question 16

Which of the following best describes the function of mycolic acid in acid-fast bacteria?

Accepted Answer

It prevents proper staining by conventional methods.

Question 17

A clinical sample is suspected to contain Nocardia. Which staining method should be used to confirm its presence and why?

Accepted Answer

Acid-fast stain, because Nocardia contains mycolic acid.

Question 18

Which of the following steps is unique to the acid-fast staining procedure compared to the Gram stain?

Accepted Answer

Use of heat to drive the primary stain into the cell wall

Question 19

In a hospital setting, why is it important to correctly identify acid-fast bacteria in patient samples?

Accepted Answer

Acid-fast bacteria, such as Mycobacterium tuberculosis, require specific treatment protocols.

Question 20

Which of the following best explains why the timing of the decolorizer is critical in the Gram stain procedure?

Accepted Answer

Over-decolorization can cause Gram-positive cells to lose the primary stain and appear Gram-negative.

Gram Stain and Acid-Fast Stain: Principles and Procedures

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