Which feature distinguishes Gram-positive bacteria from Gram-negative bacteria?

• A. Thick peptidoglycan layer in the cell wall
• B. Outer membrane with lipopolysaccharide
• C. No cell wall
• D. Mitochondria

Answer: (A)

The main idea is how the cell wall differs between these two groups. Gram-positive bacteria have a thick, multilayered peptidoglycan wall that traps the crystal violet-iodine dye during the staining process, so the cells stay purple after the decolorization step. In contrast, Gram-negative bacteria possess an outer membrane with lipopolysaccharide and a much thinner peptidoglycan layer; when alcohol is applied, the outer membrane is disrupted and the thin peptidoglycan cannot retain the dye, so these bacteria take up the counterstain and appear red or pink.

So the distinguishing feature is the thick peptidoglycan layer in the cell wall, which explains the Gram stain outcome. The presence of an outer membrane with lipopolysaccharide is a hallmark of Gram-negative bacteria, not Gram-positive, and questions about lacking a cell wall or about mitochondria aren’t the defining difference between these two groups.