Bios 351: Gram Staining

Front 1

What is the primary stain used in a gram stain?

Back 1

crystal violet

Front 2

You are given a smear of unknown bacteria and asked to do a gram stain. If the bacteria are gram-positive, what color would you observe immediately after applying the primary stain?

Back 2

purple

Front 3

You are given a smear of unknown bacteria and asked to do a gram stain. If the bacteria are gram-negative, what color would you observe immediately after applying the primary stain?

Back 3

purple

Front 4

You are given a smear of unknown bacteria and asked to do a gram stain. If the bacteria are gram-negative, what color would you observe immediately after applying the mordant?

Back 4

purple

Front 5

You are given a smear of unknown bacteria and asked to do a gram stain. If the bacteria are gram-positive, what color would you observe immediately after applying the mordant?

Back 5

purple

Front 6

What is the name of the mordant used in a gram stain?

Back 6

gram's iodine

Front 7

What is the name of the decolorizing agent used in a gram stain?

Back 7

ethanol

Front 8

What is the name of the counterstain used in a gram stain?

Back 8

safranin

Front 9

You are given a smear of unknown bacteria and asked to do a gram stain. If the bacteria are gram-positive, what color would you observe immediately after applying the decolorizer?

Back 9

purple

Front 10

You are given a smear of unknown bacteria and asked to do a gram stain. If the bacteria are gram-negative, what color would you observe immediately after applying the decolorizer?

Back 10

colorless

Front 11

You are given a smear of unknown bacteria and asked to do a gram stain. If the bacteria are gram-positive, what color would you observe immediately after applying the counterstain?

Back 11

purple

Front 12

You are given a smear of unknown bacteria and asked to do a gram stain. If the bacteria are gram-negative, what color would you observe immediately after applying the counterstain?

Back 12

red

Front 13

Describe two differences between the cell walls of gram-negative and gram-positive bacteria.

Back 13

#1: gram-positive bacteria have a thicker layer of peptidoglycan
#2: gram-negative bacteria have an outer membrane that surrounds the peptidoglycan layer

Front 14

Describe the function of the mordant used in the gram stain.

Back 14

The mordant combines with the primary stain to form a complex that cannot be washed out of gram-positive cells by the decolorizer.

Front 15

Role of a mordant?

Back 15

complexes with the crystal violet and forms an insoluble complex in gram-positive cells

Front 16

Why are thick smears bad for gram staining?

Back 16

they can entrap the primary stain, which is not removed by alcohol or acetone

Front 17

What indicates endospores in gram stain?

Back 17

clear area in the rods

Front 18

What are the 8 steps for a gram stain?

Back 18

1. crystal violets 20 sec.
2. wash 2 secs
3. gram's iodine 1 min
4. decolorize with alcohol 10-20 sec until solvent flows colorlessly
5. wash 2 secs
6. safranin 1 min
7. wash 2secs
8. blot dry