Lipopolysaccharide

Lipopolysaccharide (LPS) is a large molecule that makes up most of the outer surface of Gram-negative bacteria. It protects the cell and is strongly recognized by the immune system, where it is known as endotoxin.

LPS has three regions. Lipid A anchors it in the outer membrane and is the toxic part, a core oligosaccharide of sugars connects it to a long outer chain, the O antigen, which varies widely between strains and is used to classify them, such as in Escherichia coli O157. The molecule sits in the outer layer of the outer membrane, outside the thin peptidoglycan wall, and makes the membrane a barrier to many antibiotics.

Immune cells such as macrophages detect lipid A with the receptor TLR4 and release inflammatory signals. A little LPS helps to defend the body, but a large amount in the blood can trigger runaway inflammation, a leading cause of septic shock. Beyond inflammation, LPS is tested in medicines through a clotting assay that uses blood of the Atlantic horseshoe crab.

The LPS barrier is part of why Gram-negative bacteria resist many drugs, and the polymyxin antibiotics work by binding it. Changes to LPS, which can be spread by plasmids, contribute to antimicrobial resistance. Researchers also use LPS to study inflammation in immunology and as an ingredient in some vaccine adjuvants.

• Gram-negative Bacteria — the cells that carry it
• Macrophage — a cell that responds to it
• Antimicrobial Resistance — a problem its barrier contributes to