- 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.
* * *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.
* * *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
* * *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.