What structure do Gram-negative bacteria possess that Gram-positive bacteria do not?

Gram-negative bacteria are characterized by their unique cell wall structure, which includes an outer membrane that Gram-positive bacteria lack. This outer membrane is an essential component of the Gram-negative bacterial cell envelope, providing an additional barrier that protects the bacteria from certain antibiotics, bile salts, and detergents. It is composed mainly of lipopolysaccharides, phospholipids, and proteins, which all play critical roles in the bacteria's interaction with its environment and immune evasion.

The presence of the outer membrane is significant in determining the staining properties of bacteria in the Gram stain procedure. In contrast, Gram-positive bacteria have a much thicker peptidoglycan layer and do not possess an outer membrane. This fundamental difference in cell wall structure not only aids in laboratory identification of the bacteria but also influences their pathogenicity and susceptibility to antimicrobial agents.