Principles Of Biomedical Science Practice Exam

Which staining method differentiates gram-positive from gram-negative bacteria?

Giemsa stain Gram Stain Flagella stain Capsule stain

Correct Answer:

Gram Stain

Explanation:

Differentiating bacteria by how their cell walls take up dye is what this question tests. The Gram stain uses crystal violet and iodine to form a dye complex inside the cell wall. In Gram-positive bacteria, the thick peptidoglycan layer traps this complex, so after a brief alcohol wash they stay purple. In Gram-negative bacteria, a thinner peptidoglycan layer and an outer membrane don’t retain the dye, so they’re decolorized and then take up a counterstain (pink/red). This creates two distinct appearances that separate the two groups. Other stains target different features: Giemsa is for blood cells and certain parasites, flagella stain highlights motility structures, and capsule stain reveals a capsule around some bacteria, not Gram-type.

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