Gram Stain – A differential staining technique in which cells either stain pink (gram-negative) or purple (gram-positive) depending upon their structural type.
During a gram stain, a bacterial sample is smeared on a microscope slide and allowed to dry. The slide is then stained with a violet dye, treated with acetone-alcohol (a decoloriser) and finally counter-stained with a red dye.
Gram-positive bacteria retain the first violet dye as they have a thick layer of peptidoglycan in their cell walls creating cells which appear violet.
In gram-negative bacteria, the acetone-alcohol washes out the violet dye and the counter-stain is taken up creating cells which appear red. The cell walls of gram-negative bacteria have an outer layer of lipoprotein overlaying a thin layer of peptidoglycan.
Example process:
- slide stained with crystal violet (1 min),
- rinsed with water,
- stained with iodine (1 min),
- rinsed with water,
- destained with acetyl-choline,
- rinsed with water,
- counterstained with safranin (1 min)
