An endotoxin is a complex molecule known chemically as lipopolysaccharide (LPS). LPS is a major component of the outer membrane of Gram-negative bacteria, such as Escherichia coli and Salmonella. It is released when the bacterial cell disintegrates or sheds membrane vesicles. This molecule is a potent activator of the mammalian immune system, capable of causing a robust inflammatory response. High concentrations, especially during severe infection, can lead to symptoms like fever and, in severe cases, life-threatening septic shock.
Defining the Lipopolysaccharide Molecule
LPS is a large, amphipathic glycolipid, possessing both hydrophobic and hydrophilic regions. This structure allows it to function as a barrier on the bacterial surface. The molecule is organized into a tripartite structure: the hydrophobic Lipid A domain, the hydrophilic core oligosaccharide, and the highly variable O-antigen polysaccharide. These three distinct regions are covalently linked, forming a single macromolecule anchored to the outer leaflet of the bacterial outer membrane.
The orientation is maintained with Lipid A buried within the membrane bilayer. The sugar components (the core and the O-antigen) extend outward from the bacterial surface. This structural organization protects the bacterium from external threats, including certain antibiotics and host defense mechanisms. The distinct chemical properties of each region contribute to the overall physical integrity and biological activity of the entire bacterial cell wall.
The Toxic Anchor: Structure of Lipid A
The Lipid A domain is the innermost, hydrophobic anchor of the LPS structure and is responsible for triggering the endotoxic immune response. This section is highly conserved across Gram-negative species, reflecting its fundamental role in bacterial survival. Its chemical backbone is a $\beta-(1\rightarrow6)$-linked disaccharide composed of two glucosamine units, which are a type of amino sugar. This disaccharide is typically modified by phosphate groups at the 1 and $4’$ positions, imparting a net negative charge.
The defining feature of Lipid A is the presence of multiple fatty acid chains (acyl chains) attached via ester and amide linkages. The number and length of these chains determine the molecule’s toxicity, as they dictate interaction with the host’s primary immune receptor, Toll-like receptor 4 (TLR4).
The highly active form of Lipid A, found in bacteria like E. coli and Salmonella, is hexaacylated, possessing six fatty acid chains. These six chains include four primary $\beta$-hydroxy acyl chains attached directly to the glucosamine units. Two secondary acyl chains are often attached to the hydroxyl groups of the primary chains, completing the hexaacylated structure. This specific architecture creates the optimal geometry for binding to the TLR4/MD-2 receptor complex on host immune cells. Variations in the number of chains, such as a penta- or tetra-acylated form, can significantly reduce the inflammatory potential.
The Core and O-Antigen Components
Extending outward from the Lipid A anchor is the core polysaccharide, a non-repeating chain of sugar residues that acts as a structural bridge to the outermost O-antigen. The core is chemically divided into an inner region and an outer region, with the inner core being the most structurally conserved part of the entire sugar component.
Inner Core
This inner core is characterized by the presence of unique sugars rarely found elsewhere in nature, such as 3-Deoxy-D-manno-oct-2-ulosonic acid (KDO) and heptose (Hep). KDO residues are directly linked to the Lipid A disaccharide and are associated with the structural integrity of the outer membrane. The inner core also often contains phosphate groups, which contribute to the overall negative charge of the cell surface and assist in binding divalent cations, helping stabilize the outer membrane.
Outer Core
Moving past the inner core, the outer core region contains more common hexoses, such as glucose and galactose, and displays greater structural variability between different bacterial groups.
O-Antigen (O-Polysaccharide)
The O-antigen is the terminal, outermost, and most variable domain of the LPS molecule. It is a long, linear chain composed of multiple repeating oligosaccharide units. This region is highly hydrophilic and extends far out into the external environment, acting as the primary face of the bacterium that interacts with the host. The extreme structural diversity of the O-antigen is the basis for the serological classification of Gram-negative bacteria. The O-antigen’s length and composition determine the specific serotype of a bacterial strain, allowing organisms like E. coli to be categorized into over 160 distinct varieties.
Structural Impact on Bacterial Identity
The presence or absence of the O-antigen profoundly influences a bacterium’s surface properties, classifying LPS into two functional forms.
Smooth LPS (sLPS)
When the full O-antigen chain is present, the LPS is referred to as “Smooth” LPS (sLPS). This long, sugar-based coat forms a protective hydrophilic shield around the cell. This shield helps the bacterium evade host defenses like complement proteins and phagocytosis, often correlating with higher virulence.
Rough LPS (rLPS)
Strains producing a truncated LPS lacking the O-antigen or possessing only a shortened core structure are referred to as “Rough” LPS (rLPS). These rough strains have a more hydrophobic cell surface due to the exposure of the underlying core and Lipid A. The loss of the protective O-antigen layer makes rough bacteria significantly more susceptible to hydrophobic antibiotics and certain immune components. The structural difference also alters immune recognition pathways.