Gram-positive

Gram-positive bacteria retain the color of the crystal violet stain in the Gram stain. This is characteristic of bacteria that have a cell wall composed of a thick layer of a particular substance (called peptidologlycan). The Gram-positive bacteria include staphylococci ("staph"), streptococci ("strep"), pneumococci, and the bacterium responsible for diphtheria (Cornynebacterium diphtheriae) and anthrax (Bacillus anthracis). The Danish bacteriologist J.M.C. Gram (1853-1938) devised a method of staining bacteria using a dye called crystal (gentian) violet. Gram's method helps distinguish between different types of bacteria. The gram-staining characteristics of bacteria are denoted as positive or negative, depending upon whether the bacteria take up and retain the crystal violet stain or not.
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Refers to the ability of a bacterium to resist decolorization with alcohol after being treated with Gram crystal violet stain, imparting a violet color to the bacterium when viewed by light microscopy. This reaction is usually an indication that the outer structure of the bacterium consists of a cytoplasmic membrane surrounded by a thick, rigid bacterial cell wall composed of peptidoglycan. See Gram stain.

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gram-pos·i·tive 'gram-'päz-ət-iv, -'päz-tiv adj holding the purple dye when stained by Gram's stain used chiefly of bacteria

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gram-pos·i·tive (gram-pozґĭ-tiv) retaining the stain or resisting decolorization by alcohol in the Gram method of staining, a primary characteristic of bacteria whose cell wall is composed of a thick layer of peptidoglycan with attached teichoic acids. Cf. gram-negative.

Characteristic cell wall structures of gram-positive (left) and gram-negative (right) bacteria.


Medical dictionary. 2011.

Look at other dictionaries:

  • Gram-positive — [gram′päz′ə tiv] adj. [also g ] designating bacteria that retain the color stain: see GRAM S METHOD …   English World dictionary

  • Gram-positive — Bacillus cereus, lichtmikroskopisch, Gram Färbung: grampositiv Pseudomonas aeruginosa, lichtmikroskopisch, Gram Fä …   Deutsch Wikipedia

  • Gram-positive — Gram positif Les bactéries à Gram positif sont mises en évidence par une technique de coloration appelée coloration de Gram. Les bactéries à Gram positif apparaissent alors mauves au microscope. La technique de coloration repose sur les… …   Wikipédia en Français

  • Gram positive — Gram positif Les bactéries à Gram positif sont mises en évidence par une technique de coloration appelée coloration de Gram. Les bactéries à Gram positif apparaissent alors mauves au microscope. La technique de coloration repose sur les… …   Wikipédia en Français

  • Gram-positive — /gram poz i tiv/, adj. (often l.c.) (of bacteria) retaining the violet dye when stained by Gram s method. [1905 10; see GRAM S METHOD] * * * …   Universalium

  • Gram-positive — Gram′ pos′itive [[t]græm[/t]] adj. (often l.c.) mcr lab (of bacteria) retaining the violet dye when stained by Gram s method • Etymology: 1905–10 …   From formal English to slang

  • Gram-positive — adjective (of bacteria) being or relating to a bacterium that retains the violet stain used in Gram s method • Similar to: ↑positive, ↑confirming • Topics: ↑bacteria, ↑bacterium …   Useful english dictionary

  • gram-positive —   a. denoting a class of bacteria stainable by method invented by H. J. C. Gram,   Danish physician.    ♦ gram negative, a. denoting class of bacteria not stainable thus …   Dictionary of difficult words

  • gram-positive — adjective Date: 1907 holding the purple dye when stained by Gram s stain used chiefly of bacteria …   New Collegiate Dictionary

  • gram-positive — adjective (of a bacterium) that is stained violet by Grams method Ant: gram negative …   Wiktionary

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