What characteristic distinguishes Gram-negative bacteria from Gram-positive bacteria?

The distinguishing characteristic that differentiates Gram-negative bacteria from Gram-positive bacteria is the presence of an outer membrane and lipopolysaccharides. Gram-negative bacteria possess a thin layer of peptidoglycan surrounded by this outer membrane, which contains lipopolysaccharides (LPS). The LPS is significant because it contributes to the structural integrity of the bacteria, acts as a barrier to certain antibiotics and detergents, and plays an essential role in the immune response elicited during infection.

In contrast, Gram-positive bacteria are characterized by a thick peptidoglycan layer that retains the crystal violet stain during the Gram staining process, resulting in a purple appearance. The absence of the outer membrane in Gram-positive bacteria means they do not have lipopolysaccharides, which are critical components for Gram-negative bacteria.

Additionally, while some bacteria can form spores, this is not a feature unique to Gram-negative or Gram-positive classifications; rather, it denotes a survival mechanism used by certain species. Resistance to all antibiotics is not accurate, as different bacteria exhibit varying susceptibility depending on the type of antibiotic and specific strain characteristics. Therefore, the presence of the outer membrane and LPS is the hallmark feature that clearly distinguishes Gram-negative from Gram-positive bacteria.