Gram stain

The Gram stain is a differential staining method that classifies bacteria by how their cell envelopes retain crystal violet after decolorization. Gram-positive cells appear purple, while Gram-negative cells take up a contrasting counterstain.

Crystal violet: A basic violet dye used as the primary stain in the Gram-staining procedure. It initially colors both Gram-positive and Gram-negative bacterial cells.
Gram-positive bacteria: Bacteria whose cell envelopes have thick peptidoglycan and lack an outer membrane. Their envelopes usually retain the crystal violet–iodine complex and appear purple.
Gram-positive cocci: Spherical or roughly spherical bacteria that stain Gram-positive. Their color and shape provide a first grouping in clinical specimens.
Gram-variable bacteria: Bacteria that show inconsistent Gram-stain colors within a culture or under different conditions. Their mixed colors complicate the method’s usual positive-versus-negative interpretation.
Gram's iodine: An iodine solution used as a mordant in the Gram-staining procedure. It forms a dye–iodine complex with crystal violet inside stained cells.
Gram-negative bacteria: Bacteria whose cell envelopes have thin peptidoglycan between an inner membrane and an outer membrane. They are decolorized and then appear pink or red after counterstaining.
Gram-negative bacilli: Rod-shaped bacteria that stain Gram-negative. This common microscopy description narrows the kinds of organisms considered in a specimen.
Mycoplasma: A group of bacteria that lack a peptidoglycan cell wall. Without the usual wall structure, they do not yield a typical Gram-stain result.
Alcohol decolorization: A staining step that uses alcohol or an alcohol–acetone mixture to remove dye from some cells. It differentiates cells by whether their envelopes retain the crystal violet–iodine complex.
Bacterial cell envelope: The layers surrounding a bacterial cytoplasm, including the cytoplasmic membrane and, in many species, a cell wall or outer membrane. Its variation, especially in peptidoglycan and outer membranes, underlies Gram-stain differences.

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