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What is Gram Staining?

Anthrax gram stain

Gram staining is a common technique used to differentiate two large groups of bacteria based on their different cell wall constituents. Gram staining can not be used to stain viruses like bacteriophages due to their different composition, therefore this method has been used mainly for Bacteria and sometimes Fungi. The Gram stain procedure distinguishes between Gram-positive and Gram-negative groups by coloring these cells red or violet. 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. Alternatively, Gram-negative bacteria stain red, which is attributed to a thinner peptidoglycan wall, which does not retain the crystal violet during the decoloring process.

How Does Gram Staining Work?

Gram staining involves three processes: staining with a water-soluble dye called crystal violet, decolorization, and counterstaining, usually with safranin. Due to differences in the thickness of a peptidoglycan layer in the cell membrane between Gram-positive and Gram-negative bacteria, Gram-positive bacteria (with a thicker peptidoglycan layer) retain crystal violet stain during the decolorization process, while Gram-negative bacteria lose the crystal violet stain and are instead stained by the safranin in the final staining process. The process involves three steps:

Cells are stained with crystal violet dye. Next, a Gram's iodine solution (iodine and potassium iodide) is added to form a complex between the crystal violet and iodine. This complex is a larger molecule than the original crystal violet stain and iodine and is insoluble in water.

A decolorizer such as an ethyl alcohol or acetone is added to the sample, which dehydrates the peptidoglycan layer, shrinking and tightening it. The large crystal violet-iodine complex is not able to penetrate this tightened peptidoglycan layer and is thus trapped in the cell in Gram-positive bacteria. Conversely, the outer membrane of Gram-negative bacteria is degraded and the thinner peptidoglycan layer of Gram-negative cells is unable to retain the crystal violet-iodine complex and the color is lost.

A counterstain, such as the weakly water-soluble safranin, is added to the sample, staining it red. Since the safranin is lighter than crystal violet, it does not disrupt the purple coloration in Gram-positive cells. However, the decolorized Gram-negative cells are stained red.

How To perform gram staining

Reagents for gram staining:

  1. Crystal violet (primary stain)
  2. Iodine solution/Gram's Iodine (mordant that fixes crystal violet to the cell wall)
  3. Decolorizer (e.g. ethanol)
  4. Safranin (secondary stain)
  5. Water (preferably in a squirt bottle)

The procedure of gram staining

  1. Make a slide of the cell sample to be stained. Heat fix the sample to the slide by carefully passing the slide with a drop or small piece of sample on it through a Bunsen burner three times.
  2. Add the primary stain (crystal violet) to the sample/slide and incubate for 1 minute. Rinse slide with a gentle stream of water for a maximum of 5 seconds to remove unbound crystal violet.
  3. Add Gram's iodine for 1 minute- this is a mordant, or an agent that fixes the crystal violet to the bacterial cell wall.
  4. Rinse sample/slide with acetone or alcohol for ~3 seconds and rinse with a gentle stream of water. The alcohol will decolorize the sample if it is Gram-negative, removing the crystal violet. However, if the alcohol remains on the sample for too long, it may also decolorize Gram positive cells.
  5. Add the secondary stain, safranin, to the slide and incubate for 1 minute. Wash with a gentle stream of water for a maximum of 5 seconds. 

Interpretation of gram staining results

If the bacterium is Gram-positive, it will retain the primary stain (crystal violet) and not take the secondary stain (safranin), causing it to look violet/purple under a microscope. If the bacterium is Gram-negative, it will lose the primary stain and take the secondary stain, causing it to appear red when viewed under a microscope.
You can read more about other stains by clicking here

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