Question 1

Which of the following best explains why Gram-positive bacteria retain the crystal violet-iodine complex during the Gram stain procedure?

Accepted Answer

Their thick peptidoglycan layer traps the complex, preventing it from being washed out by ethanol.

Question 2

A clinical sample is stained using the Gram stain protocol. The cells appear pink under the microscope. Which of the following is the most likely explanation?

Accepted Answer

The bacteria are Gram-negative and have a thin peptidoglycan layer with an outer membrane.

Question 3

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

Accepted Answer

It acts as a mordant, forming a complex with crystal violet to fix the dye inside the cell wall.

Question 4

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

Accepted Answer

Crystal violet, Gram's iodine, 95% ethanol, safranin

Question 5

A student accidentally rinses their slide with water for too long after applying crystal violet. What is the most likely result?

Accepted Answer

The crystal violet may be washed away, leading to weak or absent staining of both Gram-positive and Gram-negative cells.

Question 6

Why is it important to use cultures that are 16–24 hours old for Gram staining?

Accepted Answer

Older cultures may have damaged cell walls, causing Gram-positive bacteria to appear Gram-negative.

Question 7

Which of the following best describes the role of 95% ethanol in the Gram stain procedure?

Accepted Answer

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

Question 8

A laboratory technician observes both purple and pink cells in a Bacillus culture after Gram staining. What is the correct interpretation?

Accepted Answer

Classify the culture as Gram-positive, as Bacillus can be Gram-variable.

Question 9

Which of the following is a correct safety protocol when disposing of crystal violet waste after Gram staining?

Accepted Answer

Dispose of crystal violet waste in the designated beaker, not down the sink.

Question 10

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

Accepted Answer

Their thin peptidoglycan layer and outer membrane allow the crystal violet-iodine complex to be washed out, so they take up the safranin counterstain.

Question 11

A student is preparing a broth culture from a slant. Which of the following steps is essential to avoid contamination during the transfer?

Accepted Answer

Flame the loop before and after transferring the bacteria.

Question 12

Which of the following equations best represents the calculation for total magnification when using the 100× oil immersion objective with a 10× ocular lens?

Accepted Answer

$$Total\ Magnification = Objective\ Magnification 	imes Ocular\ Magnification$$

Question 13

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

Accepted Answer

Pink, short bacillus

Question 14

A patient is diagnosed with a Gram-negative bacterial infection. Which of the following is a likely implication for antibiotic treatment?

Accepted Answer

Gram-negative bacteria may be more resistant to certain antibiotics due to their outer membrane.

Question 15

Which of the following is the correct sequence for cleaning up after a Gram stain procedure?

Accepted Answer

Dispose of crystal violet in the beaker, other stains in the sink, clean spills with Bon Ami, disinfect benchtop, wash hands.

Question 16

Which of the following best explains why a broth culture is used for a motility assay rather than a slant culture?

Accepted Answer

Broth cultures allow for the observation of bacterial movement in a liquid medium.

Question 17

A student forgets to add safranin during the Gram stain procedure. What will be the most likely appearance of Gram-negative cells under the microscope?

Accepted Answer

They will appear colorless because they do not retain the crystal violet-iodine complex.

Question 18

Which of the following is a critical reason for using bibulous paper to dry a slide after Gram staining?

Accepted Answer

Bibulous paper absorbs water without damaging the stained smear.

Question 19

Which of the following best describes the expected Gram stain result for Mycobacterium smegmatis?

Accepted Answer

Purple, bacillus with tapered ends

Question 20

A laboratory spill occurs involving a culture of Escherichia coli. What is the correct immediate action?

Accepted Answer

Notify the instructor immediately for proper disinfection.

Gram Staining and Broth Culture Techniques in Microbiology

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