Crystal Violet

Crystal violet, also known as Basic Violet 3 or Gentian Violet, is a basic, cationic aminoarylmethane dye. It is the N-hexamethylated derivative of pararosaniline and bears a lipophilic cation. It is primarily used in bacteriology and microscopy as the primary stain in Gram staining to classify and distinguish between Gram-positive and Gram-negative bacteria. It is indicator grade, exhibiting a color transition from yellow-green in acidic conditions to blue-violet in basic conditions. It also possesses antibacterial and antifungal properties.

The antimicrobial properties of crystal violet are due to free radical generation and the formation of unionized dye-bacteria complexes. It inhibits bacterial protein synthesis and cell wall formation and is especially effective against Gram-positive bacteria.

Gram staining is used to differentiate bacteria based on their cell wall structure. Gram-positive bacteria have a thicker cell wall with more peptidoglycan and less lipid content, while gram-negative bacteria have a thinner cell wall with less peptidoglycan and more lipid content. During the staining process, crystal violet is applied to the bacteria, followed by the iodine to form a complex that fixes the dye. A decolorizer is then used to remove the dye from gram-negative bacteria by dissolving their lipid layer. Gram-positive bacteria retain the dye due to the dehydration and shrinking of their cell walls. A counterstain, such as safranin or basic fuchsin, is applied to impart a pink color to decolorized gram-negative bacteria.