What characterizes gram-negative bacteria?

Gram-negative bacteria are characterized by having an outer membrane that contains lipopolysaccharides (LPS). This outer membrane is an important structural feature that differentiates them from gram-positive bacteria, which lack this outer membrane. The presence of lipopolysaccharides contributes to the structural integrity of the bacterial cell wall and plays a critical role in the bacteria's interactions with the host and immune responses. The LPS can act as an endotoxin, which is significant in the context of infections, as it can trigger strong immune responses that can lead to inflammation and sepsis.

The other options do not accurately describe gram-negative bacteria. Rather than having a thick peptidoglycan layer, gram-negative bacteria typically have a thin layer of peptidoglycan located between their inner and outer membranes. They do not stain blue in a Gram stain; instead, they take on a pink or red color due to the counterstain used after the initial crystal violet stain. Lastly, gram-negative bacteria do have a cell wall, which is composed of the thin peptidoglycan layer and the outer membrane, but they do not lack a cell wall altogether.