Question 1

Which of the following correctly lists the steps of the Gram stain procedure in order?

Accepted Answer

Stain with primary stain (crystal violet), add iodine, add decolorizer, stain with counterstain (safranin).

Question 2

Which reagent is responsible for staining Gram-negative cells pink (or red) in the Gram stain procedure?

Accepted Answer

Safranin.

Question 3

Which of the following statements about the Gram stain is FALSE?

Accepted Answer

Alcohol is used to kill the bacteria that are not stained.

Question 4

After performing a Gram stain, you observe purple, spherical bacteria under the microscope. What is the most likely Gram reaction and morphology?

Accepted Answer

Gram positive cocci.

Question 5

A scientist stains a mixed sample containing Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative). What colors will these bacteria appear after Gram staining?

Accepted Answer

Staphylococcus aureus: purple; Escherichia coli: pink.

Question 6

What is the primary function of iodine in the Gram stain procedure?

Accepted Answer

To act as a mordant and fix the crystal violet dye inside the cell wall.

Question 7

Which step in the Gram stain procedure is most critical for differentiating between Gram-positive and Gram-negative bacteria?

Accepted Answer

Application of the decolorizer (alcohol or acetone).

Question 8

If the decolorizer is left on too long during the Gram stain, what is the most likely result?

Accepted Answer

Both Gram-positive and Gram-negative cells will appear pink.

Question 9

Which structural difference in bacterial cell walls is responsible for the differential staining in the Gram stain?

Accepted Answer

Thickness of the peptidoglycan layer.

Question 10

A Gram stain is performed on a clinical sample, and the result is a mixture of purple and pink rods. What is the best interpretation?

Accepted Answer

The sample contains both Gram-positive and Gram-negative bacilli.

Question 11

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

Accepted Answer

Their thin peptidoglycan layer cannot retain the crystal violet-iodine complex.

Question 12

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

Accepted Answer

Both Gram-positive and Gram-negative cells will appear pink after counterstaining.

Question 13

Which of the following is NOT a possible reason for observing Gram-variable results (some cells purple, some pink) in a pure culture?

Accepted Answer

Using too much iodine.

Question 14

Which of the following equations best represents the chemical interaction between crystal violet and iodine in the Gram stain?

Accepted Answer

$$\mathrm{Crystal\ Violet} + \mathrm{Iodine} ightarrow \mathrm{Crystal\ Violet	ext{-}Iodine\ Complex}$$

Question 15

A clinical sample is Gram stained and observed to have pink, rod-shaped bacteria. Which of the following is the most likely identity?

Accepted Answer

Gram-negative bacilli.

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