Final answer:
The mismatched pair is B. safranin - acid dye because safranin is a basic dye, not an acid dye. By mistake, safranin is incorrectly listed as an acid dye when it should be classified as a basic dye that positively stains bacterial cell walls.
Step-by-step explanation:
You've asked which of the following pairs is mismatched:
- A. carbolfuchsin - basic dye
- B. safranin - acid dye
- C. crystal violet - basic dye
- D. iodine - mordant
- E. alcohol-acetone - decolorizer
The mismatched pair here is B. safranin - acid dye. Safranin is not an acid dye; it is actually a basic dye. Basic dyes have positively charged chromophores which bind to the negatively charged cell walls, as do methylene blue, malachite green, and crystal violet. These dyes serve as positive stains due to the attraction of the dye's positive charge to the negative charges on most bacterial cell walls. On the other hand, acidic dyes have negatively charged chromophores and do not bind as well to the cell walls, serving as negative stains. Therefore, safranin being listed as an acid dye is incorrect.
In the Gram staining procedure, iodine serves as a mordant, meaning it helps to fix or stabilize the stain by forming a complex with the primary stain, crystal violet, making it less soluble and more adherent to the cell's structures. Alcohol-acetone is used as a decolorizer; it removes the primary stain from some cells (Gram-negative) while leaving it in others (Gram-positive).
After the Gram stain procedure is completed, Gram-positive cells appear purple because their thicker peptidoglycan layer retains the crystal violet-iodine complex, even after decolorization. In contrast, Gram-negative cells appear pink due to the secondary counterstain, like safranin, because their thinner peptidoglycan layer doesn't retain the primary stain after decolorization.