Gram

Definition

Gram staining is a differential staining technique used to classify bacteria into two major groups: Gram-positive and Gram-negative, based on the chemical and physical properties of their cell walls. It involves the application of a series of dyes that reveal differences in bacterial cell wall structure.

5 Must Know Facts For Your Next Test

1. Gram staining differentiates bacteria by the composition of their cell walls, specifically peptidoglycan thickness.
2. Gram-positive bacteria retain the crystal violet stain and appear purple under a microscope.
3. Gram-negative bacteria do not retain the crystal violet stain but take up the counterstain (safranin) and appear pink or red.
4. The Gram staining process includes four steps: crystal violet application, iodine treatment, alcohol decolorization, and safranin counterstaining.
5. The technique was developed by Hans Christian Gram in 1884.

Review Questions

• What are the key structural differences between Gram-positive and Gram-negative bacteria?
• Describe the four main steps involved in the Gram staining process.
• Why do Gram-negative bacteria appear pink or red after staining?

Related terms

Peptidoglycan:

A polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of most bacteria.

Crystal Violet:

A dye used in Gram staining as the primary stain that colors all bacterial cells initially.

Safranin:

A counterstain used in Gram staining that stains decolorized cells pink or red.