Lipopolysaccharides (LPS), also known as lipoglycans, are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, act as endotoxins and elicit strong immune responses in animals. LPS is the major component of the outer membrane of Gram-negative bacteria, contributing greatly to the structural integrity of the bacteria, and protecting the membrane from certain kinds of chemical attack. LPS also increases the negative charge of the cell membrane and helps stabilize the overall membrane structure. It is of crucial importance to gram negative bacteria, whose death results if it is mutated or removed. LPS is an endotoxin, and induces a strong response from normal animal immune systems. LPS acts as the prototypical endotoxin because it binds the CD14/TLR4/MD2 receptor complex, which promotes the secretion of pro-inflammatory cytokines in many cell types, but especially in macrophages. In Immunology, the term "LPS challenge" refers to the process of exposing a subject to an LPS which may act as a toxin. LPS is also an exogenous pyrogen (external fever-inducing substance). Being of crucial importance to gram negative bacteria, these molecules make candidate targets for new antimicrobial agents. LPS comprises three parts: 1. O antigen (or O polysaccharide). 2. Core polysaccharide. 3. Lipid A. The making of LPS can be modified in order to present a specific sugar structure. Those can be recognised by either other LPS (which enables to inhibit LPS toxins) or glycosyltransferases which use those sugar structure to add more specific sugars. It has recently been shown that a specific enzyme in the intestine (alkaline phosphatase) can detoxify LPS by removing the two phosphate groups found on LPS carbohydrates [6]. This may function as an adaptive mechanism to help the host manage potentially toxic effects of gram-negative bacteria normally found in the small intestine. (from wiki). This card shows the example of LPS in E. coli with one o-antigen unit. LPS with O-antigen has been found to be a metabolite of Escherichia, Pseudomonas and Salmonella (PMID: 12045108).