Final answer:
Forgetting to stain a bacterial smear with crystal violet would prevent the differentiation between Gram-positive and Gram-negative bacteria, as the primary dye is essential for the initial purple coloration and subsequent identification steps.
Step-by-step explanation:
Forgetting to stain with crystal violet during a Gram stain procedure will affect the outcome of the experiment. Typically, crystal violet is applied to a heat-fixed smear as the primary stain, making all cells appear purple. After adding Gram's iodine (a mordant), the crystal violet-iodine complex clumps in the thick peptidoglycan layers of Gram-positive cell walls, which retains the purple color even after a decolorizing agent is applied. For Gram-negative cells, the decolorizing agent removes the crystal violet dye due to their thinner peptidoglycan layers.
Without the crystal violet staining, it would be impossible to differentiate between Gram-positive and Gram-negative bacteria, as the secondary counterstain, usually safranin, would stain all cells pink instead of just the decolorized Gram-negative cells. This crucial step is what allows for the differentiation based on the cell wall's composition and thickness, pivotal in bacterial identification and subsequent treatment decisions.