Final answer:
If the decolorizing agent is skipped in a Gram stain, Gram-negative cells will incorrectly retain the purple color from crystal violet and would not take up the pink safranin, leading to an inability to differentiate between Gram-positive and Gram-negative cells.
Step-by-step explanation:
If the decolorizing agent, typically either ethanol or an acetone/ethanol solution, is omitted during the Gram stain procedure, then both Gram-positive and Gram-negative cells will retain the purple color from the primary stain, crystal violet. Without the decolorizing step, the dye is not washed out from the Gram-negative cells. Thus, the Gram-negative cells would not take up the pink safranin counterstain and would remain purple, appearing the same as Gram-positive cells.
This error in procedure prevents the differentiation between Gram-positive and Gram-negative bacteria, which is the essential purpose of the Gram stain. Normally, after the appropriate application of the decolorizing agent, the thick peptidoglycan layer in Gram-positive cells' cell walls allows these cells to retain the purple color, while the thinner peptidoglycan layer in Gram-negative cells does not. Therefore, in a correct staining procedure, the Gram-positive cells remain purple while Gram-negative cells would appear pink due to the uptake of the safranin after decolorization.