What primarily distinguishes the structure of Gram-negative bacteria?

The presence of an outer membrane is a key characteristic that primarily distinguishes Gram-negative bacteria from Gram-positive bacteria. In Gram-negative bacteria, the outer membrane is an additional barrier that lies outside the thin peptidoglycan layer, which is generally around 2 to 7 nanometers thick. This outer membrane consists of lipopolysaccharides (LPS) and serves multiple functions: it acts as a protective barrier against certain antibiotics and detergents, aids in the maintenance of the structural integrity of the bacteria, and can also contribute to pathogenicity. The outer membrane's unique composition and structure have significant implications for the behavior and treatment of Gram-negative bacterial infections.

In contrast, Gram-positive bacteria are characterized by a much thicker peptidoglycan layer and lack this outer membrane. This difference in structure accounts for their distinctive responses to Gram staining, where Gram-negative bacteria do not retain the primary stain, resulting in a pink coloration due to the counterstain. Understanding the structural differences between Gram-negative and Gram-positive bacteria is important for the development of antibiotics and treatment strategies, as they respond differently based on their cellular structures.