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How does the Gram stain distinguish gram-positive bacteria from gram-negative bacteria?
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Microbiology
Photomicrograph of Streptococcus pyogenes, a bacterium that can cause scarlet fever. (Magnified about 900Γ.)
The Gram stain distinguishes between bacteria based on fundamental differences in their cell wall structure. The process involves staining cells with crystal violet, treating them with iodine, and then applying an organic solvent, such as alcohol or acetone.1
- Gram-positive bacteria: These possess a thick, meshlike layer of peptidoglycan that is not easily penetrated by the solvent. Consequently, they retain the purple crystal violet stain.12
- Gram-negative bacteria: These have a much thinner peptidoglycan layer and an additional outer membrane. The solvent easily washes the dye out of these cells. A counterstain, typically safranin, is then applied, which turns the decolorized gram-negative cells red.12
This technique, devised by Hans Christian Gram, remains a vital tool in microbiology for the identification and characterization of bacterial species.1