Which of the following best describes a feature of Gram-negative bacteria?

Gram-negative bacteria are characterized by having a unique cell wall structure that includes two distinct layers. The outer layer is composed of an outer membrane containing lipopolysaccharides, while the inner layer consists of a thin peptidoglycan layer. This dual-layer structure is crucial as it contributes to the bacteria's overall stability and provides a barrier to certain antibiotics and detergents.

When subjected to the Gram stain procedure, Gram-negative bacteria do not retain the crystal violet dye and thus do not appear purple; instead, they take up the counterstain (usually safranin), resulting in a pink appearance. This is distinctly different from Gram-positive bacteria, which have a thick peptidoglycan layer and retain the crystal violet dye. The presence or absence of a cell wall is also not applicable to Gram-negative bacteria as they do possess a cell wall, albeit structured differently compared to Gram-positive bacteria. Additionally, Gram-negative bacteria are not always pathogenic; many are part of the normal flora and are not harmful to humans. Therefore, the best way to describe a feature of Gram-negative bacteria is their characteristic two-layered cell wall structure.