Gram-negative Bacteria: Structure, Entry Routes and Endotoxin Quiz

Option E is correct: the typical Gram-negative envelope has two membranes, with thin peptidoglycan between them. Peptidoglycan is a mesh of sugar chains joined by short peptides, and it helps the bacterium keep its shape. It sits in the periplasm, the space between the inner and outer membranes. The outer membrane adds a barrier that can limit antibiotic entry, while the inner membrane encloses the cell contents. Picture a thin supporting mesh sandwiched between two membranes.

The thick wall in B belongs to the usual Gram-positive arrangement, where there is no outer membrane. A single bilayer without the second membrane or its porins (A) also misses the defining Gram-negative structure. A capsule is an additional surface coat, often made of sugars; it cannot substitute for the membranes as proposed in C. Teichoic and lipoteichoic acids (D), chains of repeating units associated with the Gram-positive wall, likewise point to a different envelope arrangement.