In the Gram stain, which sequence correctly lists the major steps?

In the Gram stain, which sequence correctly lists the major steps?

• A. Crystal violet, alcohol, safranin, iodine
• B. Crystal violet, iodine, decolorizer, safranin
• C. Iodine, crystal violet, decolorizer, safranin
• D. Safranin, crystal violet, decolorizer, iodine

Answer: (B)

Gram staining relies on how the dye interacts with cell wall structure to distinguish different bacteria, using a primary stain, a mordant, a decolorizer, and a counterstain in a specific order. The correct sequence starts with crystal violet to stain all cells, then uses iodine as a mordant to form a larger crystal violet–iodine complex that is trapped in the cell wall. The decolorizer (ethanol or acetone) then differentiates the cells by washing away the CV–I complex from cells with a thinner peptidoglycan layer or an outer membrane. Finally, safranin provides a contrasting color to the now colorless Gram-negative cells, while Gram-positive cells remain purple due to retention of the CV–I complex. Other sequences misplace the mordant, decolorizer, or counterstain, which disrupts the chemical interactions and prevents reliable differentiation.

Gram staining relies on how the dye interacts with cell wall structure to distinguish different bacteria, using a primary stain, a mordant, a decolorizer, and a counterstain in a specific order. The correct sequence starts with crystal violet to stain all cells, then uses iodine as a mordant to form a larger crystal violet–iodine complex that is trapped in the cell wall. The decolorizer (ethanol or acetone) then differentiates the cells by washing away the CV–I complex from cells with a thinner peptidoglycan layer or an outer membrane. Finally, safranin provides a contrasting color to the now colorless Gram-negative cells, while Gram-positive cells remain purple due to retention of the CV–I complex. Other sequences misplace the mordant, decolorizer, or counterstain, which disrupts the chemical interactions and prevents reliable differentiation.