Biology 3.4 - Gram Staining

Gram staining = a method of staining the cell walls of bacteria to aid their identification.

Prior to gram staining, all bacteria are colourless. Then...

Gram-positive bacteria = purple.

Gram-negative bacteria = purple. 

• The cell wall of bacteria is a 3D network of peptidoglycan. The cross-linking of these molecules provides strength and gives bacteria their unique shape.
• The cell wall also protects agains swelling/bursting or lysis caused by the osmotic uptake of water. 
• Gram-positive bacteria have the basic cell wall structure. HOWEVER, gram-negative bacteria have an additional outer layer of lipopolysaccharide. 

How does gram staining work?

Steps:

1. Crystal violet, a basic dye, is added to the bacteria. This binds to the peptidoglycan layer, so all bacteria stain…