Q: Which of the following equations 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 difference in Gram stain results between Gram-positive and Gram-negative bacteria?

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

If the decolorizer (ethyl alcohol) is left on a Gram stain preparation for too long, what is the most likely result?

Accepted Answer

Gram-positive cells will appear pink.

Question 4

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

Accepted Answer

Decolorization

Question 5

Why do Mycobacterium species require the acid-fast stain rather than the Gram stain?

Accepted Answer

Their cell wall contains a thick layer of mycolic acid.

Question 6

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

Accepted Answer

Carbolfuchsin

Question 7

After performing an acid-fast stain, what color will non-acid-fast cells appear under the microscope?

Accepted Answer

Blue

Question 8

Which of the following is a critical factor to consider when preparing a smear for Gram staining?

Accepted Answer

The age of the bacterial culture

Question 9

A clinical sample is stained and observed to have pink cells after Gram staining. What does this indicate about the bacteria?

Accepted Answer

They are Gram-negative.

Question 10

Which chemical is used as the decolorizer in the Gram stain procedure?

Accepted Answer

Ethyl alcohol

Question 11

In the acid-fast stain, why is heat applied during the primary staining step?

Accepted Answer

To allow carbolfuchsin to penetrate the mycolic acid layer

Question 12

Which genus of bacteria is most commonly identified using the acid-fast stain?

Accepted Answer

Mycobacterium

Question 13

A laboratory technician accidentally skips the iodine step in the Gram stain procedure. What is the most likely outcome?

Accepted Answer

Gram-positive cells will appear pink.

Question 14

Which of the following best explains why Gram-negative bacteria lose the crystal violet stain during decolorization?

Accepted Answer

They have a thin peptidoglycan layer and an outer membrane.

Question 15

What is the function of the counterstain in both Gram and acid-fast staining procedures?

Accepted Answer

To stain cells that did not retain the primary stain

Question 16

Which of the following equations 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 17

A patient sample is suspected to contain Mycobacterium tuberculosis. Which staining procedure should be performed and what result would confirm the presence of acid-fast bacteria?

Accepted Answer

Acid-fast stain; red or pink cells

Question 18

Which of the following is NOT a factor that can affect the accuracy of Gram stain results?

Accepted Answer

Type of microscope

Question 19

In a mixed culture containing both Staphylococcus aureus and Mycobacterium smegmatis, which staining method would best differentiate these two organisms?

Accepted Answer

Acid-fast stain, because Mycobacterium will appear red and Staphylococcus will appear blue

Question 20

Which of the following best describes the appearance of Gram-positive and Gram-negative bacteria after the complete Gram stain procedure?

Accepted Answer

Gram-positive: purple; Gram-negative: pink

Gram Stain and Acid-Fast Stain: Principles and Laboratory Procedures

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