Lipopolysaccharide (LPS), now more commonly known as endotoxin, is a collective term for components of the outermost membrane of the cell envelope of gram-negative bacteria, such as E. coli and Salmonella with a common structural architecture. Lipopolysaccharides are large molecules consisting of three parts: an outer core polysaccharide termed the O-antigen, an inner core oligosaccharide and Lipid A (from which toxicity is largely derived), all covalently linked. In current terminology, the term endotoxin is often used synonymously with LPS, although there are a few endotoxins (in the original sense of toxins that are inside the bacterial cell that are released when the cell disintegrates) that are not related to LPS, such as the so-called delta endotoxin proteins produced by Bacillus thuringiensis.
Lipopolysaccharides can have substantial impacts on human health, primarily through interactions with the immune system. LPS is a potent activator of the immune system and is a pyrogen (agent that causes fever). In severe cases, LPS can trigger a brisk host response and multiple types of acute organ failure which can lead to septic shock. In lower levels and over a longer time period, there is evidence LPS may play an important and harmful role in autoimmunity, obesity, depression, and cellular senescence.