Which structural feature best distinguishes Gram-positive bacteria from Gram-negative bacteria?
A. Presence of pili for bacterial conjugation
B. Presence of lipopolysaccharide (LPS) in the outer membrane
C. Presence of a thick peptidoglycan layer (20–80 nm) in the cell wall
D. Possession of flagella for motility
Explanation
Distinguishing Gram-Positive from Gram-Negative Bacteria
Structural Basis of Gram Staining
The Gram stain differentiates bacteria based on cell wall composition, not staining chemistry alone. The key discriminator is the thickness and composition of the peptidoglycan layer.
Comparative Cell Wall Architecture
Table
Feature
Gram-Positive
Gram-Negative
Peptidoglycan thickness
20–80 nm (thick, multilayered)
5–10 nm (thin, single layer)
Outer membrane
Absent
Present (lipid bilayer with LPS)
Teichoic acids
Present (in peptidoglycan)
Absent
Lipopolysaccharide (LPS)
Absent
Present in outer membrane
Periplasmic space
Absent or minimal
Prominent (20–40 nm)
Why Peptidoglycan Thickness is the Best Discriminator
Key Point
The thick peptidoglycan layer in Gram-positive bacteria retains the crystal violet–iodine complex during alcohol decolorization, appearing purple. Gram-negative bacteria have thin peptidoglycan and an outer membrane that is disrupted by alcohol, releasing the dye and allowing safranin counterstain to enter, appearing pink.
High-YieldNEET PG
This structural difference is the fundamental basis for the Gram stain classification and is the single most reliable distinguishing feature at the ultrastructural level.
Clinical Pearl
The thick peptidoglycan of Gram-positive bacteria also makes them more susceptible to penicillin and other beta-lactams, which cross-link peptidoglycan. Gram-negative bacteria are protected by their outer membrane, requiring different antibiotic strategies.
Why Other Features Are Not Primary Discriminators
LPS (option B): Present only in Gram-negative bacteria, but is a consequence of the outer membrane structure, not the primary distinguishing feature.
Flagella (option C): Present in both Gram-positive and Gram-negative bacteria; not discriminatory.
Pili (option D): Present in both Gram-positive and Gram-negative bacteria; not discriminatory.
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