Gram Staining

Question 1

What is Gram Staining ?

Answer 1

Differential staining of bacteria based on (as we now know) differences in their cell walls.

Question 2

How do we prepare gram staining ?

Answer 2

1. Applicaton of Crystal violet (Purple)
2. Application of Iodine(mordant)
3. Alchol wash decolourization
4. Application of Safrain

Question 3

What is the difference between gram negative and gram positive bacteria ?

Answer 3

Gram positive – a lot of peptidoglycan, only one membrane

Gram negative – two membranes, a very small peptidoglycan layer in the per plasm

Question 4

Difference between resistant ?

Answer 4

Gram negative bacteria more resistant to antibiotics than gram positive bacteria because gram negative bacteria have a cell wall with an outer membrane which protects them from the antibiotic .

Question 5

Difference in terms of Structure ?

Answer 5

Gram Positive Cell walls

• Thick Peptidoglycan
• Teichoic Acids

Gram negative cell walls :

• Thin Peptidoglycan
• Outer membrane

Persiplamic space

Question 6

What about the gram negative cell walls ?

Answer 6

Perisplasm between outer membrane and the plasma membrane contains thin layer of peptidoglycan

Outer membrane made out of polysaccharide,lipoporteins and phospholipids .

Question 7

Features of Gram negative Outer membrane ?

Answer 7

Protects from phagocytes, complement and some antibiotics

Made of lipopolysaccharide (LPS)

O polysaccharide functions as antigen
(e.g., E. coli O157:H7)

Lipid A is an endotoxin embedded in the top layer

Porins (proteins) form channels through membrane

Question 8

Why do bacteria stain purple ?

Answer 8

Gram positive bacteria stain violet due to the presence of a thick layer of peptidoglycan in their cell walls, which retains the crystal violet these cells are stained with.