Question 1

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

Accepted Answer

Decolorization with ethanol

Question 2

What is the primary reason Gram-negative bacteria appear pink after the Gram stain procedure?

Accepted Answer

Their outer membrane is dissolved by alcohol, allowing safranin to stain the cells.

Question 3

If you forget to add the counterstain (safranin) during the Gram stain procedure, what color will Gram-negative bacteria appear under the microscope?

Accepted Answer

Colorless

Question 4

Which of the following best explains why Gram-positive bacteria retain the crystal violet-iodine complex after decolorization?

Accepted Answer

Their thick peptidoglycan layer is dehydrated by alcohol, trapping the complex.

Question 5

Which of the following is NOT a difference between Gram-positive and Gram-negative bacteria?

Accepted Answer

Ability to form endospores

Question 6

A clinical sample is stained using the Gram stain and both purple cocci and pink rods are observed. What is the most likely explanation?

Accepted Answer

The culture is contaminated with both Gram-positive and Gram-negative bacteria.

Question 7

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

Accepted Answer

Crystal violet, Gram’s iodine, ethanol, safranin

Question 8

Why is the Gram stain considered clinically significant?

Accepted Answer

It helps guide antibiotic therapy by identifying bacterial cell wall type.

Question 9

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

Accepted Answer

Gram-positive bacteria stain purple.

Question 10

A student performs a Gram stain on a 72-hour-old culture and observes both pink and purple cells. What is the most likely reason for this result?

Accepted Answer

The culture is too old, causing some Gram-positive cells to lose their ability to retain the crystal violet-iodine complex.

Question 11

Which chemical acts as the mordant in the Gram stain procedure?

Accepted Answer

Gram’s iodine

Question 12

What would happen if you reversed the order of ethanol and Gram’s iodine in the Gram stain procedure?

Accepted Answer

The differentiation between Gram-positive and Gram-negative bacteria would be lost.

Question 13

Which of the following best describes the appearance of Staphylococcus epidermidis after a correct Gram stain?

Accepted Answer

Purple cocci

Question 14

During the Gram stain, what is the function of ethanol?

Accepted Answer

It decolorizes Gram-negative cells by dissolving their outer membrane.

Question 15

Which of the following equations best represents the formation of the crystal violet-iodine complex?

Accepted Answer

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

Question 16

Which of the following best explains why Gram-negative bacteria are more susceptible to antibiotics that target the outer membrane?

Accepted Answer

Their outer membrane is easily disrupted by chemicals such as ethanol.

Question 17

If a student observes only pink cells after performing a Gram stain on a mixed culture, which step was most likely performed incorrectly?

Accepted Answer

Crystal violet was not added.

Question 18

Which of the following best describes the clinical utility of the Gram stain?

Accepted Answer

It allows for rapid preliminary identification of bacterial pathogens.

Question 19

Which of the following is a plausible reason for observing both Gram-positive and Gram-negative cells in a pure culture after Gram staining?

Accepted Answer

The culture is old and some cells have lost their ability to retain the crystal violet-iodine complex.

Question 20

Which of the following best describes the appearance of Escherichia coli after a correct Gram stain?

Accepted Answer

Pink rods

Gram Stain: Principles, Procedure, and Interpretation

Study Guide - Practice Questions