Final answer:
In the process of Gram staining, Gram-positive bacteria appear purple due to their thick cell wall that retains the crystal violet dye. Gram-negative bacteria, which have a thinner cell wall and an outer membrane, do not retain the crystal violet and appear pink after being counterstained with safranin.
Step-by-step explanation:
In the classification of bacteria using the Gram stain technique, Gram-positive bacteria are characterized by a purple color after the staining process. This coloration is due to the presence of a thick peptidoglycan layer in the cell walls of Gram-positive bacteria, which retains the crystal violet dye used in the process. On the other hand, Gram-negative bacteria appear pink after the process because they have a thinner peptidoglycan layer and an outer membrane that does not retain the crystal violet dye, instead taking up the counterstain, usually safranin.
For example, in a Gram-stained specimen, the violet rod-shaped cells forming chains are the Gram-positive bacteria Bacillus cereus, and the small, pink cells are the Gram-negative bacteria Escherichia coli. When performing a Gram stain, Gram-positive cells resist decolorization and remain purple, while the dye is released from Gram-negative cells, which are then counterstained pink.