The Gram stain differentiates bacteria based upon structural differences of the bacterial cell wall. Which of the following best describes the key difference between Gram-positive and Gram-negative cell walls?

• A. Both Gram-positive and Gram-negative bacteria have identical cell wall structures.
• B. Gram-positive bacteria have a thin peptidoglycan layer and an outer membrane.
• C. Gram-negative bacteria have a thick peptidoglycan layer and no outer membrane.
• D. Gram-positive bacteria have a thick peptidoglycan layer and lack an outer membrane.

Answer: (D)

Verified step by step guidance

1. Understand that the Gram stain is a differential staining technique used to classify bacteria into two major groups: Gram-positive and Gram-negative, based on their cell wall structure.
2. Recall that the key structural component involved in this differentiation is the peptidoglycan layer, which provides rigidity and shape to the bacterial cell wall.
3. Note that Gram-positive bacteria have a thick peptidoglycan layer, which retains the crystal violet stain during the Gram staining process, making them appear purple under a microscope.
4. Recognize that Gram-negative bacteria have a much thinner peptidoglycan layer and possess an additional outer membrane external to the peptidoglycan, which does not retain the crystal violet stain but takes up the counterstain (safranin), appearing pink or red.
5. Conclude that the key difference is that Gram-positive bacteria have a thick peptidoglycan layer and lack an outer membrane, whereas Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane.