Final answer:
Crystal violet is a stain that penetrates bacterial cell membranes and binds strongly to negative cell constituents, used in Gram staining to differentiate between Gram-positive (purple) and Gram-negative (pink) bacteria.
Step-by-step explanation:
The type of stain that penetrates the cell membrane of bacteria and therefore has a strong affinity for the negative constituents (membrane, DNA) of the cell is crystal violet. This stain is employed in the Gram staining technique, a differential staining method that classifies bacteria into two groups based on their cell wall composition. During the Gram stain procedure, all cells initially take up the crystal violet stain and appear purple. However, the application of alcohol causes Gram-negative cells to lose this stain, thereby decolorizing them. Afterward, the cells are counterstained with safranin, which stains only the decolorized Gram-negative cells pink, whereas Gram-positive cells retain the crystal violet stain and remain purple.