Final answer:
Gram-negative bacteria stain pink in the Gram stain process due to their thin cell wall that allows the crystal violet dye to escape after decolorization. Gram-positive bacteria, on the other hand, have thicker cell walls that retain the purple dye even after the decolorization step.
Step-by-step explanation:
Gram-negative bacteria appear pink after a Gram stain is applied. This contrasting color is due to the unique structural differences in the cell walls of Gram-positive and Gram-negative bacteria.
When conducting a Gram stain, crystal violet dye is used initially and both types of cells pick up the color. Then, Gram's iodine is applied, which helps to trap the dye inside the cells. The next step involves applying alcohol, which acts as a decolorizer.
Gram-positive bacteria have a thick cell wall that traps the crystal violet-iodine complex, preventing the dye from being washed out by the alcohol. As a result, Gram-positive cells resist decolorization and remain purple.
In contrast, Gram-negative bacteria have a thinner cell wall with an additional outer membrane. The alcohol wash dissolves this outer membrane and allows the crystal violet dye to escape.
After decolorization, a counterstain called safranin is applied, which stains the now colorless Gram-negative cells, causing them to appear pink. This highlights the differences between the two groups – the red, safranin-dyed Gram-negative cells and the purple, crystal violet-stained Gram-positive cells.