Q: Which of the following equations best represents the formation of the crystal violet-iodine complex during Gram staining?

$$\text{Crystal Violet} + \text{Iodine} \rightarrow \text{Crystal Violet-Iodine Complex}$$

Question 1

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

Accepted Answer

Application of 95% ethyl alcohol, because it determines whether the primary stain is retained or removed based on cell wall structure.

Question 2

In the Gram stain procedure, what is the primary function of Gram's iodine?

Accepted Answer

To act as a mordant, forming an insoluble complex with the crystal violet.

Question 3

A student forgets to apply the decolorizing agent during the Gram stain procedure. What will be the likely result when observing a mixed culture of Gram-positive and Gram-negative bacteria?

Accepted Answer

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

Question 4

What is the role of heat fixing a bacterial smear before staining?

Accepted Answer

To kill the bacteria and adhere them to the slide.

Question 5

Which of the following best explains why Gram-negative bacteria appear pink after Gram staining?

Accepted Answer

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

Question 6

Refer to the provided image. Based on the morphology and color, what is the most likely Gram reaction and shape of the bacteria shown?

Accepted Answer

Gram-positive bacilli

Question 7

Why is it important to limit the decolorization step to 15-20 seconds during the Gram stain procedure?

Accepted Answer

Over-decolorization can remove the primary stain from Gram-positive cells, causing them to appear Gram-negative.

Question 8

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

Accepted Answer

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

Question 9

A clinical sample is stained and observed under oil immersion. The cells appear as pink rods. Which organism is most likely present?

Accepted Answer

Escherichia coli

Question 10

Which of the following best describes the function of safranin in the Gram stain procedure?

Accepted Answer

It counterstains cells that have been decolorized, making Gram-negative cells visible.

Question 11

If a Gram-positive culture is more than 24 hours old, what is a likely outcome when performing a Gram stain?

Accepted Answer

The cells may appear Gram-negative due to weakened cell walls.

Question 12

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

Accepted Answer

Their thick peptidoglycan layer traps the complex, preventing its removal by alcohol.

Question 13

During the Gram stain, why is it important to blot the slide dry with bibulous paper before examining under oil immersion?

Accepted Answer

To prevent water from interfering with the oil immersion lens and image clarity.

Question 14

Which of the following best describes the sequence of reagents used in the Gram stain procedure?

Accepted Answer

Crystal violet, iodine, alcohol, safranin

Question 15

A laboratory technician observes purple, rod-shaped bacteria under the microscope after Gram staining. Which of the following is the most likely identification?

Accepted Answer

Gram-positive bacilli

Gram Staining: Principles, Procedure, and Interpretation

Study Guide - Practice Questions

Study Guide - Flashcards