Question 1

Which of the following best describes the main structural difference between Gram-positive and Gram-negative bacterial cell walls?

Accepted Answer

Gram-positive bacteria have a thick peptidoglycan layer, while Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane.

Question 2

During the Gram stain procedure, what is the primary function of iodine?

Accepted Answer

It acts as a mordant, forming a complex with crystal violet to enhance retention in the cell wall.

Question 3

What is the most likely result if you over-decolorize a Gram stain?

Accepted Answer

Gram-positive cells may appear Gram-negative (red/pink).

Question 4

Why is heat fixation NOT used in the capsule staining procedure?

Accepted Answer

Heat destroys the capsule, making it invisible under the microscope.

Question 5

Which of the following is the correct order of reagents in the Gram stain procedure?

Accepted Answer

Crystal violet, iodine, decolorizer, safranin

Question 6

A student prepares a bacterial smear that is too thick. What is the most likely effect on the Gram stain result?

Accepted Answer

Gram-negative bacteria may appear Gram-positive due to insufficient decolorization.

Question 7

What is the main advantage conferred by a bacterial capsule?

Accepted Answer

It helps bacteria resist phagocytosis by host immune cells.

Question 8

In capsule staining, why is dried skim milk powder added to the bacterial culture?

Accepted Answer

It provides a proteinaceous background that helps visualize the capsule as a clear halo.

Question 9

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

Accepted Answer

The alcohol/acetone dissolves the outer membrane and the thin peptidoglycan layer cannot retain the complex.

Question 10

A Gram stain is performed on a mixed culture of Staphylococcus epidermidis and Escherichia coli. What colors would you expect to see under the microscope for each organism?

Accepted Answer

Staphylococcus epidermidis will appear purple and Escherichia coli will appear pink/red.

Question 11

Which of the following is NOT a reason for a false Gram stain result?

Accepted Answer

Using a fresh, young bacterial culture.

Question 12

What is the role of the decolorizer in the Gram stain procedure?

Accepted Answer

It removes the crystal violet-iodine complex from Gram-negative cells.

Question 13

Which of the following best describes a differential stain?

Accepted Answer

A stain that distinguishes between different types of bacteria based on cellular structures.

Question 14

If a student forgets to add iodine during the Gram stain procedure, what is the most likely outcome?

Accepted Answer

Both Gram-positive and Gram-negative cells may appear pink/red after counterstaining.

Question 15

Which of the following steps is unique to the capsule staining procedure compared to the Gram stain?

Accepted Answer

Using copper sulfate to decolorize the capsule.

Question 16

What is the main reason for gently blotting the slide dry with bibulous paper after staining?

Accepted Answer

To remove excess water without disturbing the stained smear.

Question 17

Which of the following best describes the appearance of a bacterial capsule after capsule staining?

Accepted Answer

A clear, colorless halo surrounding the stained bacterial cell and background.

Question 18

Why is it important to use the correct amount of water when preparing a bacterial smear?

Accepted Answer

Too much water can prevent proper drying and lead to poor staining results.

Question 19

Which of the following is a function of the bacterial capsule in the environment?

Accepted Answer

Prevents drying and helps bacteria adhere to surfaces.

Question 20

What is the expected result if a Gram stain is performed on an old culture of a Gram-positive bacterium?

Accepted Answer

The cells may appear Gram-negative due to deterioration of the cell wall.

Gram Stain and Capsule Stain: Microbiology Lab Study Guide

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