Which statement best describes Gram-positive versus Gram-negative regarding outer membranes?

• A. Gram-negative have an outer membrane and LPS; Gram-positive have thick peptidoglycan without an outer membrane.
• B. Gram-positive have an outer membrane with LPS; Gram-negative have none.
• C. Gram-positive have thick peptidoglycan and no outer membrane; Gram-negative have thin peptidoglycan and an outer membrane with LPS.
• D. Gram-positive lack peptidoglycan; Gram-negative lack outer membrane.

Answer: (C)

The main idea is that the outer membrane is a defining feature: Gram-positive bacteria lack an outer membrane and have a thick peptidoglycan layer, while Gram-negative bacteria have a thin peptidoglycan layer plus an outer membrane that contains lipopolysaccharide (LPS). This outer membrane in Gram-negatives acts as an additional barrier and includes LPS, which is a recognizable endotoxin component.

This description fits best because Gram-positive bacteria are characterized by a robust, thick peptidoglycan cell wall and no outer membrane, whereas Gram-negative bacteria have a two-membrane envelope with a thinner peptidoglycan layer sandwiched in the periplasm and an outer membrane rich in LPS. The other statements would imply Gram-positive have an outer membrane or Gram-negative lack one, or misstate the presence of peptidoglycan, which conflicts with the established cell-envelope structures.