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

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

• A. Retains crystal violet and thick peptidoglycan
• B. Retains safranin counterstain; outer membrane with LPS; thin peptidoglycan
• C. Has no outer membrane
• D. Always forms endospores

Answer: (B)

Understanding the staining difference starts with how the cell envelope is organized. Gram-negative bacteria have an outer membrane that contains lipopolysaccharide (LPS), a relatively thin peptidoglycan layer, and a periplasmic space. During Gram staining, the alcohol decolorizes the outer membrane and washes away the crystal violet–iodine complex, so these cells take up the red/pink safranin counterstain and appear pink. Gram-positive bacteria, with a thick peptidoglycan wall and no outer membrane, retain the crystal violet–iodine complex and stay purple after decolorization. The description that matches Gram-negative bacteria is the one mentioning the outer membrane with LPS, a thin peptidoglycan layer, and uptake of the safranin counterstain. The other statements describe Gram-positive features or unrelated traits.

Understanding the staining difference starts with how the cell envelope is organized. Gram-negative bacteria have an outer membrane that contains lipopolysaccharide (LPS), a relatively thin peptidoglycan layer, and a periplasmic space. During Gram staining, the alcohol decolorizes the outer membrane and washes away the crystal violet–iodine complex, so these cells take up the red/pink safranin counterstain and appear pink. Gram-positive bacteria, with a thick peptidoglycan wall and no outer membrane, retain the crystal violet–iodine complex and stay purple after decolorization. The description that matches Gram-negative bacteria is the one mentioning the outer membrane with LPS, a thin peptidoglycan layer, and uptake of the safranin counterstain. The other statements describe Gram-positive features or unrelated traits.