What color do Gram-negative bacteria typically appear after completing a Gram stain and viewing under light microscopy?

What color do Gram-negative bacteria typically appear after completing a Gram stain and viewing under light microscopy?

• A. Purple
• B. Red
• C. Pink
• D. Colorless

Answer: (C)

Grain staining separates bacteria by how their cell walls hold dye. The Gram stain uses crystal violet with iodine to lock the dye in. In Gram-positive bacteria, the thick peptidoglycan layer traps this dye, so the cells stay purple. Gram-negative bacteria, with a thinner peptidoglycan layer and an outer membrane, lose the purple dye during the decolorization step. Afterward, a counterstain (safranin) is applied, which those bacteria take up, giving a pink/red appearance under light microscopy. So the typical final color for Gram-negative bacteria is pink.

Grain staining separates bacteria by how their cell walls hold dye. The Gram stain uses crystal violet with iodine to lock the dye in. In Gram-positive bacteria, the thick peptidoglycan layer traps this dye, so the cells stay purple. Gram-negative bacteria, with a thinner peptidoglycan layer and an outer membrane, lose the purple dye during the decolorization step. Afterward, a counterstain (safranin) is applied, which those bacteria take up, giving a pink/red appearance under light microscopy. So the typical final color for Gram-negative bacteria is pink.