Question 1

Which of the following best explains why basic (cationic) stains are commonly used to visualize bacterial cells?

Accepted Answer

Basic stains are attracted to the negatively charged bacterial cell surface, enhancing contrast.

Question 2

During the preparation of a bacterial smear, why is heat fixing an essential step?

Accepted Answer

It kills the cells and adheres them to the slide, preventing them from washing off during staining.

Question 3

Which of the following is a consequence of preparing a bacterial smear that is too thick?

Accepted Answer

The morphology of individual cells will be difficult to observe due to poor light transmission.

Question 4

In the Gram staining procedure, what is the primary function of the iodine (mordant) step?

Accepted Answer

To fix the crystal violet dye inside the cell wall, forming a crystal violet-iodine complex.

Question 5

A microbiologist performs a Gram stain on a mixed culture and observes purple cocci and pink rods. What is the most accurate interpretation?

Accepted Answer

The cocci are Gram-positive and the rods are Gram-negative.

Question 6

Which structural feature is responsible for the retention of crystal violet in Gram-positive bacteria during the alcohol wash step?

Accepted Answer

Thick peptidoglycan layer cross-linked by tetrapeptides

Question 7

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

Accepted Answer

Their thin peptidoglycan layer allows the crystal violet-iodine complex to be washed out by alcohol, so they take up the safranin counterstain.

Question 8

Which of the following best describes the principle behind negative staining?

Accepted Answer

The stain colors the background, leaving the cells unstained and visible as clear shapes.

Question 9

A patient sample is stained using the acid-fast procedure. Red rods are observed under the microscope. What does this indicate?

Accepted Answer

The bacteria are acid-fast, likely Mycobacterium species.

Question 10

Which component of the Gram-negative cell wall is responsible for its toxicity and ability to trigger strong immune responses in humans?

Accepted Answer

Lipid A portion of lipopolysaccharide (LPS)

Question 11

Which of the following equations best represents the cross-linking of peptidoglycan in Gram-positive bacteria?

Accepted Answer

$$	ext{NAM} - 	ext{tetrapeptide} - 	ext{cross-bridge} - 	ext{tetrapeptide} - 	ext{NAM}$$

Question 12

Which of the following is NOT a characteristic of Gram-positive bacteria?

Accepted Answer

Presence of an outer membrane containing LPS

Question 13

A laboratory technician forgets to apply the iodine step during Gram staining. What is the most likely result?

Accepted Answer

Gram-positive cells may lose the crystal violet during alcohol wash and appear pink, similar to Gram-negative cells.

Question 14

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

Accepted Answer

Alcohol wash (decolorization)

Question 15

Why are Mycobacterium species resistant to many antibiotics and phagocytosis?

Accepted Answer

They possess a waxy cell wall containing mycolic acid.

Question 16

Which of the following best describes the role of a mordant in simple staining?

Accepted Answer

It helps fix the primary stain to the cell or increases the size of the stained structure.

Question 17

Which of the following stains would be most appropriate for negative staining?

Accepted Answer

Nigrosin

Question 18

A clinical sample is stained with carbol fuchsin, heated, washed with acid alcohol, and then counterstained with methylene blue. Blue cells are observed. What does this indicate?

Accepted Answer

The cells are non-acid-fast.

Question 19

Which of the following best explains why Gram-negative bacteria are more susceptible to physical disruption but less susceptible to penicillin than Gram-positive bacteria?

Accepted Answer

They have a thin peptidoglycan layer and an outer membrane that protects against penicillin.

Question 20

Which of the following best describes the arrangement and morphology that can be determined by simple staining?

Accepted Answer

Shape (cocci, bacilli, spirilla) and arrangement (chains, clusters, pairs, tetrads)

Bacterial Staining Techniques and Cell Wall Structure

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