Question 1

Which of the following best explains why bacterial cells remain unstained in a negative stain procedure using nigrosine?

Accepted Answer

The negatively charged chromophore of the dye is repelled by the negatively charged bacterial cell surface.

Question 2

Which of the following is an advantage of negative staining over simple staining?

Accepted Answer

Negative staining does not require heat fixation, reducing cell distortion.

Question 3

Which of the following dyes is commonly used in negative staining?

Accepted Answer

Nigrosine

Question 4

A student prepares a negative stain but observes that the bacterial cells appear distorted. What is the most likely cause?

Accepted Answer

The slide was heat-fixed before staining.

Question 5

Which of the following can be determined from a negative stain? Select all that apply.

Accepted Answer

Cell shape

Question 6

During the negative staining procedure, why is it important to allow the slide to air dry rather than heat-fix it?

Accepted Answer

Heat-fixing can distort the true morphology of the cells.

Question 7

Which of the following statements about acidic dyes used in negative staining is FALSE?

Accepted Answer

They have a positive charge.

Question 8

A student observes a slide prepared with a negative stain and sees clear, round cells arranged in clusters. What is the most likely identity of these cells?

Accepted Answer

Staphylococcus species

Question 9

Which step in the negative staining procedure is critical for ensuring an even smear and accurate results?

Accepted Answer

Spreading the drop of dye and bacteria in one continuous, fluid motion with a second slide.

Question 10

If a student uses Congo red instead of nigrosine for a negative stain, what will be the result?

Accepted Answer

The background will be stained, and the cells will remain clear.

Question 11

Which of the following best describes the appearance of bacterial cells under the microscope after a successful negative stain?

Accepted Answer

Clear cells against a dark background

Question 12

Why is it important to avoid dragging the spreader slide back and forth multiple times during the negative staining procedure?

Accepted Answer

It can cause uneven smears and disrupt cell morphology.

Question 13

Which of the following best explains why negative staining is useful for observing bacterial capsules?

Accepted Answer

Capsules do not take up acidic dyes and appear as clear halos around cells.

Question 14

A student accidentally heat-fixes a slide before performing a negative stain. What is the most likely outcome?

Accepted Answer

The cells may appear distorted and not reflect their true morphology.

Question 15

Which of the following is NOT a characteristic of a negative stain?

Accepted Answer

It uses a basic dye with a positive charge.

Negative Staining in Microbiology: Principles, Procedure, and Interpretation

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