Lab 9 - Gram stain

Question 1

what is the purpose of Gram stain?

Answer 1

It helps us identify bacteria, and in medicine this can help doctors prescribe the correct medications to treat infections

Question 2

most bacteria can be divided into two groups based on their reactions. what are they?

Answer 2

Gram-positive and Gram-negative
(structural and differential stain)

Question 3

there are a few groups if bacteria that do not stain with the Gram stain including what?

Answer 3

mycoplasma and mycobacteria

Question 4

what are the staining steps?

Answer 4

1. Crystal violet (primary stain) - both cells stain purple.
2. Iodine (mordant) clumper - both cells remain purple.
3. Alcohol (decolorizer) - Gram-positive cells remain purple; Gram-negative cells become colorless.
4. Safranin (counterstain) - Gram-positive cells remain purple; Gram-negative cells appear pink.

Question 5

what forms when the mordant clumper iodine is added? what is the purpose of the clumper?

Answer 5

the CV-I complex forms; it turns the smear a darker purple. the purpose is so that the cell keeps the stain.

Question 6

what do the following indicate:
Purple:
Red/Pink:
Rod-shaped:
Spherical:

Answer 6

Gram-positive
Gram-negative
Rods
Cocci

Question 7

T/F: Bacteria in agar cultures produce more concentrated growth; whereas, bacteria grown in both cultures are more spread out

Answer 7

True

Question 8

What type of Gram stain does Bacillus subtilis show up as?

Answer 8

Gram-positive rods

Question 9

What type of Gram stain does E. coli show up as?

Answer 9

Gram-negative rods

Question 10

what happens to a Gram-positive cell wall before decolorization?

Answer 10

The large crystal violet-iodine complexes form within the cytoplasm as a result of the first two steps of the Gram stain.

Question 11

what happens to a Gram-positive cell wall after decolorization?

Answer 11

The decolorizing agent dehydrates the thick layer of the peptidoglycan, causing it to become a tight mesh-like structure that prevents the crystal violet-iodine complexes from being washed out of the cell.

Question 12

what happens to a Gram-negative cell wall before decolorization?

Answer 12

The large crystal violet-iodine complexes form within the cytoplasm as a result of the first two steps of the Gram-stain.

Question 13

what happens to a Gram-negative cell wall after decolorization?

Answer 13

The decolorizing agent damages the outer membrane. The thin PG layer cannot prevent the crystal violet-iodine complexes from being washed out of the cell.

Question 14

what structure does the Gram stain actually stain?

Answer 14

the stain crosses the PG to stain the actual cell, and not the PG

Question 15

Gram-stain is a _____ stain, which involves what two types of stains?

Answer 15

differential stain; involves a primary stain and a counterstain

Question 16

What organism is an example of Gram-positive cocci:

Answer 16

Staphylococcus epidermis

Question 17

What organism is an example of Gram-negative rods:

Answer 17

E. coli

Question 18

What are two reasons that Gram-positive cells can appear Gram-negative?

Answer 18

1. If the smear was decolorized for too long.
2. If the cells are dead or dying.

Question 19

What are two reasons that Gram-negative cells can appear Gram-positive?

Answer 19

1. If the smear was not decolorized long enough.
2. If the cells are too thickly smeared, and they trap some of the crystal violet dye between the cell layers.

Question 20

what are common examples of Gram-positive cells?

Answer 20

-Bacillus
-Staphylococcus
-Streptococcus

Question 21

what are common examples of Gram-negative cells?

Answer 21

-E. coli
-Pseudomonas
-Salmonella