2026 Microbiology Lab Practical Practice Exam – Complete Test Prep

Gram staining distinguishes bacteria by cell wall structure. The thick peptidoglycan layer in Gram-positive bacteria traps the crystal violet-iodine complex, so they stay purple after decolorization. Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane that lets the dye wash out, and when the counterstain is applied, they take up the pink/red color. So pink indicates Gram-negative cells. Purple indicates Gram-positive, and blue/green are not typical end colors in this standard stain.