endotoxins

Endotoxin is the lipid A component of lipopolysaccharide (LPS), a structural component of the outer membrane of gram-negative bacteria (e.g., E. coli, Neisseria, Salmonella, Pseudomonas). LPS consists of three parts: O antigen (polysaccharide, serotype-specific), core polysaccharide, and lipid A (the toxic component embedded in the membrane). Endotoxin is released when bacteria lyse (e.g., after complement-mediated lysis or antibiotic treatment) rather than actively secreted like exotoxins.

Mechanism: Lipid A is recognized by the innate immune system via binding to LPS-binding protein (LBP), which presents it to CD14 on macrophages, leading to activation of TLR4/MD-2 complex. This triggers NF-κB activation and massive release of pro-inflammatory cytokines—TNF-α, IL-1, IL-6, IL-8, and nitric oxide—causing the clinical manifestations of sepsis: fever, hypotension, vascular permeability, disseminated intravascular coagulation (DIC) via tissue factor induction, and potentially multi-organ failure and septic shock.

Key distinguishing features from exotoxins: Endotoxins are heat-stable (withstand 100°C for 1 hour), weakly immunogenic, do not have specific receptors (act through TLR4 broadly), have no specific antidote, and cannot be converted to toxoids for vaccines. They are generally less potent (require higher doses to cause effect, measured in micrograms) compared to exotoxins (potent in nanogram quantities).

Clinical significance: Endotoxemia is central to the pathophysiology of gram-negative sepsis and septic shock. The Limulus amebocyte lysate (LAL) assay detects endotoxin and is used to test for pyrogen contamination in pharmaceuticals and medical devices. Clinically, gram-negative bacteremia (e.g., from E. coli, Klebsiella, Pseudomonas) is a major cause of septic shock, characterized by warm shock (early, vasodilation) progressing to cold shock, hypotension refractory to fluids, and high mortality without prompt antibiotic and supportive therapy.