Gram stain: 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.

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 (specifically, peptidologlycan containing teichoic and lipoteichoic acid complexed to the peptidoglycan).

The Gram-positive bacteria include staphylococci ("staph"), streptococci ("strep"), pneumococci, and the bacterium responsible for diphtheria (Cornynebacterium diphtheriae) and anthrax (Bacillus anthracis).

Gram-negative bacteria lose the crystal violet stain (and take the color of the red counterstain) in Gram's method of staining. This is characteristic of bacteria that have a cell wall composed of a thin layer of a particular substance (specifically, peptidoglycan covered by an outer membrane of lipoprotein and lipopolysaccharide containing endotoxin).

The Gram-negative bacteria include most of the bacteria normally found in the gastrointestinal tract that can be responsible for disease as well as gonococci (venereal disease) and meningococci (bacterial meningitis). The organisms responsible for cholera and bubonic plague are Gram-negative.

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