lipopolysakkaridin

Lipopolysakkaridin (lipopolysaccharide, LPS) is a large molecule that forms part of the outer membrane of most Gram-negative bacteria. It consists of lipid A, a core polysaccharide, and an O antigen polysaccharide; the O antigen is highly variable, giving strain-specific serological properties. Lipid A anchors the molecule in the outer membrane and is the component primarily responsible for endotoxic effects.

Biological role and effects: LPS contributes to the barrier function of the outer membrane and helps maintain integrity against chemical stress. In hosts, LPS is recognized by the innate immune system through Toll-like receptor 4 (TLR4) in complex with MD-2, leading to the production of proinflammatory cytokines. High systemic levels can cause fever, hypotension, and septic shock.

Variations and biosynthesis: Some bacteria produce rough LPS lacking the O antigen; modifications of lipid A can alter toxicity and immune recognition. LPS biosynthesis involves multiple genes and pathways to assemble lipid A, core, and O antigen, followed by transport to the outer membrane via the Lpt system.

Clinical and research relevance: LPS is used as a model endotoxin and in assays such as the Limulus amebocyte lysate (LAL) test for endotoxin detection. Detoxified derivatives such as monophosphoryl lipid A (MPLA) are used as vaccine adjuvants to enhance immune responses with reduced toxicity.

See also: Endotoxin, Gram-negative bacteria, Toll-like receptor, Vaccine adjuvant.