A bacterial cell is interpreted as gram-positive when it forms purple insoluble crystal violet-iodine complexes within the cell during the Gram stain.

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Gram-positive bacteria have a thick peptidoglycan layer in their cell walls that traps the crystal violet-iodine complex, causing the cells to appear purple under the microscope. Gram-negative bacteria have thinner peptidoglycan layers and an extra outer membrane that prevents the complex from forming, so they appear pink after the counterstain is applied.

A Bacterial Cell Is Interpreted as Gram-Positive When It Forms