Lab 3- Gram Stain & General Staining Procedures

Question 1

Using sterile technique to transfer a small amount of bacterial culture to a clean microscope
slide and mixing it with a small “loopful” of water is called creating a what?

Answer 1

Culture emulsion

Question 2

Which of the following is the slowest step in the Gram stain procedure?

Answer 2

Allowing the emulsion to air dry

Question 3

Which of the following is the correct sequence for the Gram stain procedure?

Answer 3

Heat fix  Crystal violet primary stain  Iodine mordant  decolorize with acetone /
alcohol  Safranin counterstain  View microscopically

Question 4

A simple stain allows microbiologists to do which of the following?

Answer 4

Classify the microbes based on their unique staining characteristics

Question 5

A differential stain allows microbiologists to do which of the following?

Answer 5

Classify the microbes based on their unique staining characteristics

Question 6

According to what you have learned in class, the bacterial cell type which possess an inner and
outer phospholid bilayer, lipopolysaccharide membrane components and a comparatively thin
peptidoglycan layer would be expected to stain how?

Answer 6

Pink, Gram negative

Question 7

Approximately how long should you decolorize in the Gram stain procedure?

Answer 7

1-3 seconds as needed

Question 8

What is the differential step in the Gram stain procedure?

Answer 8

Decolorizing

Question 9

Forgetting to treat with Gram’s iodine would likely cause which of the following to occur?

Answer 9

The crystal violet would be decolorized from both Gram + and – cells alike, resulting in pink
cells

Question 10

Which of the following are common Gram stain errors?

Answer 10

• Heat fixing the incorrect surface of the slide and incinerating the microbes
• Treating with primary stain too long
• Counterstaining too long

Question 11

Which of the following microbial cell wall components accounts for the differential
characteristic of the Gram stain procedure?

Answer 11

Thin or thick peptidoglycan

Question 12

According to your reading and in-class discussions which of the following is NOT an important
factor to consider when interpreting Gram stain results?

Answer 12

Age of the staining solutions being used

Question 13

A simple stain performed with Safranin will likely yield all pink-colored cells, therefore these cells
can be classified as Gram-negative cells

Answer 13

F

Question 14

Gram staining is a crucial, common first step used to identify microbes in the medical setting

Answer 14

True

Question 15

Failing to decolorize a fixed bacterial emulsion would result in a false Gram negative result.

Answer 15

False

Question 16

You can tell approximately how well your Gram stain turned out by viewing the microscope
slide with your unaided eye after you’ve completed the staining procedure

Answer 16

False

Question 17

Generally speaking, you want the heaviest microbial smear possible on your microscope slide
prior to performing the Gram stain procedure.

Answer 17

False

Question 18

What is the gram stain result for Escherichia coli

Answer 18

negative

Question 19

What is the gram stain result for Bacillus subtilis

Answer 19

positive

Question 20

What is the gram stain result for Micrococcus luteus

Answer 20

positive

Question 21

What is the gram stain result for Corynebacterium xerosis

Answer 21

positive

Question 22

What is the gram stain result for Staphylococcus epidermidis

Answer 22

postive

Question 23

List two non-biological contaminants that might make interpreting your Gram stain results
difficult

Answer 23

Growth Substance, Environmental Pollutants

Question 24

Forgetting to treat with Gram’s iodine would likely cause which of the following to occur

Answer 24

False Gram negatives