Skip to main content

Gram's stain

Gram's stain, laboratory staining technique that distinguishes between two groups of bacteria by the identification of differences in the structure of their cell walls. The Gram stain, named after its developer, Danish bacteriologist Christian Gram, has become an important tool in bacterial taxonomy, distinguishing between so-called gram-positive bacteria, which remain colored after the staining procedure, and gram-negative bacteria, which do not retain dye. In the staining technique, cells on a microscope slide are heat-fixed (killed) and stained with a basic dye, crystal violet, which stains all bacterial cells blue; then they are treated with an iodine-potassium iodide solution that allows the iodine to enter the cells and form a water-insoluble complex with the crystal violet dye. The cells are treated with alcohol or acetone solvent in which the iodine-crystal violet complex is soluble. Following solvent treatment, only gram-positive cells remain stained, possibly because of their thick cell wall, which is not permeable to solvent. After the staining procedure, cells are treated with a counterstain, i.e., a red acidic dye such as safranin or acid fuchsin, in order to make gram-negative (decolorized) cells visible. Counterstained gram-negative cells appear red, and gram-positive cells remain blue. Although the cell walls of gram-negative and gram-positive bacteria are similar in chemical composition, the cell wall of gram-negative bacteria is a thin layer sandwiched between an outer lipid-containing cell envelope and the inner cell membrane, whereas the gram-positive cell wall is much thicker, lacks the cell envelope, and contains additional substances, such as teichoic acids, polymers composed of glycerol or ribitol. The difference in reactivity between gram-positive and gram-negative bacteria is linked with differences in physiological properties of the two groups. Gram-positive bacteria are generally more sensitive to growth inhibition by dyes, halogens, many antibiotics, and to attack by phagocytosis (see endocytosis), and are more resistant to digestion by the enzymes pepsin and trypsin and enzymes in animal sera.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Gram's stain." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 23 Apr. 2017 <http://www.encyclopedia.com>.

"Gram's stain." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (April 23, 2017). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/grams-stain

"Gram's stain." The Columbia Encyclopedia, 6th ed.. . Retrieved April 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/grams-stain

Gram's stain

Gram's stain A staining method used to differentiate bacteria. The bacterial sample is smeared on a microscope slide, stained with a violet dye, treated with acetone-alcohol (a decolourizer), and finally counterstained with a red dye. Gram-positive bacteria retain the first dye, appearing blue-black under the microscope; such bacteria have a thick layer of peptidoglycan in their cell walls. In Gram-negative bacteria, the acetone-alcohol washes out the violet dye and the counterstain is taken up, the cells appearing red. The cell walls of these bacteria have an outer layer of lipoprotein overlying a thin layer of peptidoglycan. The stain is named after the Danish bacteriologist H. C. J. Gram (1853–1938), who first described the technique (since modified) in 1884.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Gram's stain." A Dictionary of Biology. . Encyclopedia.com. 23 Apr. 2017 <http://www.encyclopedia.com>.

"Gram's stain." A Dictionary of Biology. . Encyclopedia.com. (April 23, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/grams-stain-0

"Gram's stain." A Dictionary of Biology. . Retrieved April 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/grams-stain-0

Gram's stain

Gram's stain (gramz) n. a method of staining bacterial cells, used as a primary means of identification. The bacterial cells are stained with a violet dye, treated with decolorizer (e.g. alcohol), and then counterstained with red dye. Gram-negative bacteria lose the initial stain but take up the counterstain, so that they appear red microscopically. Gram-positive bacteria retain the initial stain, appearing violet microscopically. [ H. C. J. Gram (1853–1938), Danish physician]

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Gram's stain." A Dictionary of Nursing. . Encyclopedia.com. 23 Apr. 2017 <http://www.encyclopedia.com>.

"Gram's stain." A Dictionary of Nursing. . Encyclopedia.com. (April 23, 2017). http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/grams-stain

"Gram's stain." A Dictionary of Nursing. . Retrieved April 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/caregiving/dictionaries-thesauruses-pictures-and-press-releases/grams-stain

Gram's stain

Gram's stain The staining procedure used to determine the Gram reaction of bacteria.

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Gram's stain." A Dictionary of Plant Sciences. . Encyclopedia.com. 23 Apr. 2017 <http://www.encyclopedia.com>.

"Gram's stain." A Dictionary of Plant Sciences. . Encyclopedia.com. (April 23, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/grams-stain

"Gram's stain." A Dictionary of Plant Sciences. . Retrieved April 23, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/grams-stain