By Marcia Spear
Updated Mar 24, 2022
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The Gram stain is a foundational microbiology technique that distinguishes Gram‑positive from Gram‑negative bacteria based on their cell wall structure and resulting color. Acetone alcohol plays a pivotal role as the decolorizing agent, allowing the final color differentiation to reveal bacterial identity.
After preparing a bacterial smear, crystal violet is applied for 30 seconds. At this stage, both Gram‑positive and Gram‑negative cells appear uniformly purple. A brief wash with water removes excess dye, though some crystal violet remains bound to the thick peptidoglycan layers of Gram‑positive cells.
Iodine is added for one minute, functioning as a mordant that binds to crystal violet and forms an insoluble crystal violet‑iodine complex. This complex adheres strongly to the robust peptidoglycan of Gram‑positive bacteria, securing the primary stain without a subsequent water rinse.
Acetone or ethyl alcohol is used for 10–20 seconds to remove the crystal violet‑iodine complex from Gram‑negative cells. The alcohol dissolves the outer lipid membrane, allowing the dye to escape the thinner peptidoglycan layer. After the run‑off is colorless, a water rinse stops the decolorizing action. Gram‑positive cells retain the purple stain, while Gram‑negative cells become colorless.
Safranin is applied for approximately one minute to provide contrast, staining the now colorless Gram‑negative bacteria pink. The darker crystal violet mask ensures Gram‑positive cells remain purple. A final water rinse removes excess safranin.
