Crystal violet is a chemical called an aniline dye, also going by the name gentian violet. It is the first stain used in the Gram stain process. The stain is a positive ion that forms ionic bonds with negatively charged parts of the bacterial cell. The purple color of Gram + bacteria is due to the presence of crystal violet bound to the cells following the completed staining procedure.
Crystal violet is toxic to many groups of microbes – bacteria, fungi, and parasitic worms. Gram positive bacteria are more sensitive to its presence than are Gram negatives, which is why it is included as an inhibitor in some bacteriological media used to isolate Gram negative bacteria.