Endotoxin and Lipopolysaccharide

Lipopolysaccharide (LPS), or endotoxin, is the dominant molecule of the outer leaflet of the Gram-negative bacterial outer membrane. Unlike secreted exotoxins, it is a structural component of the cell that becomes biologically active when bacteria are lysed: its lipid A anchor is a potent trigger of innate immunity, and in excess it drives the systemic inflammation of Gram-negative sepsis.

Definition

Lipopolysaccharide is a glycolipid of the Gram-negative outer membrane composed of a membrane-anchoring lipid A, a core oligosaccharide, and a variable O-antigen polysaccharide; its lipid A portion is the endotoxic principle that activates innate immune signalling.

Scope

This topic covers the architecture of LPS (lipid A, core oligosaccharide, and O-antigen), how its lipid A moiety is sensed by the innate immune system, and why endotoxin causes systemic inflammation. It is a reference topic in bacterial pathogenesis and does not address the clinical management of sepsis.

Core questions

• What are the three structural regions of LPS, and what does each contribute?
• How does the host recognize lipid A, and through what signalling pathway?
• Why does endotoxin produce systemic inflammation rather than localized injury?

Key concepts

• Lipid A (endotoxic principle)
• Core oligosaccharide
• O-antigen
• TLR4-MD2 recognition
• Innate immune activation
• Endotoxin and Gram-negative sepsis

Mechanisms

LPS has three regions: lipid A, a glucosamine-based phospholipid that anchors the molecule and carries its toxic activity; a core oligosaccharide; and the O-antigen, a repeating polysaccharide that varies between strains and contributes to serological diversity. The host detects lipid A through the Toll-like receptor 4 complex with MD-2, with LPS-binding protein and CD14 delivering the ligand; engagement triggers signalling that induces pro-inflammatory cytokines. This recognition is protective at low exposure but, when LPS is abundant (as in disseminated Gram-negative infection), the same pathway drives an excessive systemic inflammatory response associated with septic shock.

Clinical relevance

Endotoxin's recognition by innate immunity explains why Gram-negative bacteraemia can produce profound systemic inflammation, and LPS structure underlies bacterial serotyping. This entry summarizes endotoxin biology for reference and is not guidance for diagnosing or treating sepsis.

History

The concept of a heat-stable endotoxin distinct from secreted exotoxins dates to Richard Pfeiffer in the 1890s. The twentieth century resolved its chemistry as lipopolysaccharide and localized its toxicity to lipid A, and the turn of the century identified Toll-like receptor 4 as the host sensor of endotoxin, work recognized in studies of innate immune signalling.

Key figures

• Christian R. H. Raetz
• Bruce Beutler
• Ernst Th. Rietschel

Seminal works

• raetz-whitfield-2002
• beutler-rietschel-2003

Frequently asked questions

Why is LPS called an endotoxin rather than an exotoxin?

It is not secreted but is an integral structural part of the Gram-negative outer membrane, released mainly when the bacterium is damaged or lysed, in contrast to exotoxins which are actively exported proteins.

Which part of LPS is responsible for its toxicity?

The lipid A anchor is the endotoxic principle; it is the portion recognized by the host's TLR4-MD2 complex and the trigger of the inflammatory response.