Endotoxin, also known as lipopolysaccharides (LPS) are large molecules found in the outer membrane of Gram-negative bacteria, consisting of a lipid and a polysaccharide.

The lipid component, called Lipid A, is a phosphorylated glucosamine disaccharide decorated with multiple fatty acids. The fatty acid chain anchors the endotoxin molecule into the bacterial membrane.

The polysaccharide comprises two parts: core oligosaccharide and O antigen (or O polysaccharide).

• The core domain contains an oligosaccharide that attaches directly to lipid A, and commonly contains sugars such as heptose and keto-deoxyoctulosonate. The core chain is highly diverse among bacterial species and even within strains of species.
• The O antigen is a repetitive glycan polymer attaching to the core oligosaccharide. It also comprises the outermost domain of the endotoxin molecule, being exposed on the very out surface of the bacterial cell and serving as a target for recognition by host antibodies.