Question 1

Which staining technique is used to differentiate between Gram-positive and Gram-negative bacteria, and what is the primary difference in their cell wall structure that this technique exploits?

Accepted Answer

Gram stain; thickness of peptidoglycan layer

Question 2

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

Accepted Answer

It serves as a mordant to fix the crystal violet to the cell wall.

Question 3

A microbiologist performs a Gram stain and observes purple-stained cells under the microscope. What can be inferred about these cells?

Accepted Answer

They are Gram-positive and have a thick peptidoglycan layer.

Question 4

If a bacterial sample is over-decolorized during the Gram stain procedure, what is the most likely result?

Accepted Answer

Gram-positive cells will appear pink/red.

Question 5

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

Accepted Answer

Decolorization with alcohol or acetone

Question 6

Why do Gram-negative bacteria appear pink after Gram staining?

Accepted Answer

Their thin peptidoglycan layer does not retain the crystal violet after decolorization, allowing safranin to stain them.

Question 7

A student performs a Gram stain on a mixed culture and observes both purple and pink cells. What does this indicate about the sample?

Accepted Answer

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

Question 8

Which of the following best explains why Gram-positive bacteria are more resistant to physical disruption than Gram-negative bacteria?

Accepted Answer

Gram-positive bacteria have a thick peptidoglycan layer.

Question 9

In a clinical setting, why is it important to determine whether a bacterial infection is caused by a Gram-positive or Gram-negative organism?

Accepted Answer

Gram-positive and Gram-negative bacteria respond differently to antibiotics due to differences in cell wall structure.

Question 10

Which chemical is used as the counterstain in the Gram stain procedure, and what color does it impart to Gram-negative cells?

Accepted Answer

Safranin; pink/red

Question 11

A laboratory technician accidentally skips the iodine step during Gram staining. What is the most likely outcome?

Accepted Answer

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

Question 12

Which of the following equations best represents the relationship between the concentration of crystal violet ($$C_{CV}) $$and the intensity of staining ($$I) in a $$properly performed Gram stain?

Accepted Answer

$$I = k 	imes C_{CV}$$, where $$k is a $$constant

Question 13

Which of the following is a common error that can lead to false Gram-negative results when staining Gram-positive bacteria?

Accepted Answer

Over-decolorization

Question 14

A patient sample is stained and shows Gram-negative rods. Which of the following is most likely true about the cell wall structure of these bacteria?

Accepted Answer

They have a thin peptidoglycan layer and an outer membrane.

Question 15

Which of the following best describes the role of alcohol in the Gram stain procedure?

Accepted Answer

It decolorizes Gram-negative cells by dissolving the outer membrane and removing the crystal violet-iodine complex.

Question 16

If a microbiologist wants to confirm the presence of Gram-positive cocci in a sample, which combination of shape and color should they look for under the microscope after Gram staining?

Accepted Answer

Spherical, purple cells

Question 17

Which of the following is NOT a reason why Gram staining is considered a differential staining technique?

Accepted Answer

It stains all bacteria the same color.

Question 18

A sample is stained and observed to have both purple and pink cells. If the sample was known to contain only one species, what is the most likely explanation?

Accepted Answer

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

Question 19

Which of the following best explains why Gram-negative bacteria are generally more resistant to certain antibiotics than Gram-positive bacteria?

Accepted Answer

Gram-negative bacteria have an outer membrane that acts as a barrier to many antibiotics.

Microbial Growth and Differential Staining Techniques

Study Guide - Practice Questions

Study Guide - Flashcards