bacteria staining violet or purple in the Gram staining procedure after treatment with Gram's stain

(of bacteria) being or relating to a bacterium that retains the violet stain used in Gram's method

refers to bacteria that have a single cell wall that does not contain lipopolysaccharide; these bacteria retain a violet stain when rinsed.

Gram-positive bacteria, such as anthrax, 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).

Gram-positive bacteria are classified as bacteria that retain a crystal violet dye during the Gram stain process. Gram-positive bacteria will appear blue or violet under a microscope, whereas Gram-negative bacteria will appear red or pink. The difference in classification is largely based on a difference in the bacteria's cell wall structure.