What is the main characteristic of gram negative bacteria compared to gram positive bacteria?

Gram-negative bacteria are primarily characterized by the presence of an outer membrane, which is a significant distinguishing feature compared to gram-positive bacteria. This outer membrane is composed of lipopolysaccharides and serves as an additional protective barrier. It not only helps to protect the bacteria from certain antibiotics and detergents but also plays a critical role in the immune response, as some components can act as endotoxins that can induce strong immune reactions in a host.

In contrast, gram-positive bacteria have a much thicker peptidoglycan layer that retains the crystal violet stain used in the Gram staining process, which is why they appear purple under a microscope. The absence of an outer membrane in gram-positive bacteria means they lack this additional layer of protection, making them more susceptible to certain types of antibiotics, particularly those that target cell wall synthesis.