What staining method distinguishes between two kinds of bacterial cell walls?

What staining method distinguishes between two kinds of bacterial cell walls?

• A. Capsule stain
• B. Acid-fast stain
• C. Gram stain
• D. Spore stain

Answer: (C)

The key idea is how a cell wall’s structure affects dye retention in differential staining. The Gram stain uses a dye-iodine complex that is trapped by a thick peptidoglycan layer found in Gram-positive bacteria, so these cells stay purple after the alcohol decolorization step. Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane; the alcohol disrupts this outer membrane and washes away the dye, allowing the counterstain to color them pink. That combination—purple for thick-walled Gram-positive, pink for thinner-walled Gram-negative—is how this stain distinguishes the two kinds of cell walls. Other stains target different features: capsule staining highlights capsules, acid-fast staining detects mycolic acids, and spore staining reveals endospores.

The key idea is how a cell wall’s structure affects dye retention in differential staining. The Gram stain uses a dye-iodine complex that is trapped by a thick peptidoglycan layer found in Gram-positive bacteria, so these cells stay purple after the alcohol decolorization step. Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane; the alcohol disrupts this outer membrane and washes away the dye, allowing the counterstain to color them pink. That combination—purple for thick-walled Gram-positive, pink for thinner-walled Gram-negative—is how this stain distinguishes the two kinds of cell walls. Other stains target different features: capsule staining highlights capsules, acid-fast staining detects mycolic acids, and spore staining reveals endospores.