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Lipopolysaccharides, often abbreviated as LPS, are large molecules consisting of a lipid and a polysaccharide. They are typically found in the outer membrane of Gram-negative bacteria. LPS plays a crucial role in the structure and integrity of the bacterial cell wall, acting as a barrier against certain antibiotics and environmental insults.

The LPS molecule is composed of three main regions: lipid A, a core polysaccharide, and an O-antigen. Lipid A is the hydrophobic anchor embedded within the bacterial outer membrane. The core polysaccharide is a more conserved region linked to lipid A, and the O-antigen is a variable polysaccharide chain extending outwards from the cell surface. The O-antigen is highly diverse among different bacterial species and strains, contributing to serological identification and acting as a target for the host immune system.

In the context of host-pathogen interactions, LPS is a potent immunostimulant. It is recognized by specific receptors on immune cells, such as Toll-like receptor 4 (TLR4), triggering a cascade of inflammatory responses. While this response is vital for clearing bacterial infections, excessive or uncontrolled LPS signaling can lead to harmful inflammation, septic shock, and organ damage. Therefore, LPS is a key factor in the pathogenesis of many Gram-negative bacterial infections.