What makes a Gram-positive bacterium different from a Gram-negative bacterium?

• A. Contains an outer membrane with LPS
• B. Lacks peptidoglycan entirely
• C. Retains crystal violet and thicker peptidoglycan layer
• D. Has a thinner peptidoglycan layer

Answer: (C)

The main idea here is how the cell wall structure determines the Gram stain result. Gram-positive bacteria have a thick peptidoglycan layer that traps the crystal violet-iodine complex during the staining process, so the dye is retained and the cells appear purple. This thick, dense wall also contributes to their lack of an outer membrane, distinguishing them from Gram-negative bacteria.

In contrast, Gram-negative bacteria possess a thinner peptidoglycan layer plus an outer membrane rich in lipopolysaccharide; during decolorization the dye is washed out and they take up a counterstain, appearing pink. So the statement that best captures the difference is that Gram-positive bacteria retain crystal violet and have a thicker peptidoglycan layer. The other descriptions point to Gram-negative features or atypical cell walls.