The Peptidoglycan Cell Wall and the Gram Stain
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Bacterial walls contain peptidoglycan; the Gram stain classifies a bacterium by whether its wall retains the crystal-violet stain, which peptidoglycan thickness determines.
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A thick peptidoglycan layer retains the crystal violet stain, turning purple; a thin layer loses it, turning pink at the counterstain.
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The Gram stain sorts bacteria by wall structure — it says nothing about shape, metabolism, or virulence.