Lipopolysaccharide (LPS) is the major component of the outer membrane of Gram-negative bacteria. Lipopolysaccharide is localized in the outer layer of the membrane and is, in noncapsulated strains, exposed on the cell surface. Within Gram-negative bacteria, the membrane lipopolysaccharides protect the bacterium against the action of bile salts and lipophilic antibiotics.
Lipopolysaccharides are heat stable endotoxins and have long been recognized as a key factor in septic shock (septicemia) in humans and, more generally, in inducing a strong immune response in normal mammalian cells. The lipid A moiety has been identified as critical to the endotoxin activity of lipopolysaccharide. This was demonstrated by finding identical bioactive results, including endotoxic activity, between synthetic and natural-sourced E. coli lipid A preparations. The active receptor for lipopolysaccharide has been identified as the CD14/TLR4/MD2 receptor complex, which promotes the secretion of proinflammatory cytokines including tumor necrosis factor-α and interleukin-1. While the lipid A component is primarily responsible for immune response activation, the polysaccharide component of Salmonella enterica LPS is also necessary for NF-κB activation.
Lipopolysaccharide preparations have been used in research for the elucidation of LPS structure, metabolism, immunology, physiology, toxicity, and biosynthesis. They have also been used to induce synthesis and secretion of growth promoting factors such as interleukins. Because of its connection to septicemia, lipopolysaccharide has been studied to identify possible targets for antibodies and inhibitors to LPS biosynthesis.
Lipopolysaccharides can be prepared by extraction from TCA, phenol, or phenol-chloroform-petroleum ether (for rough strains). TCA extracted lipopolysaccharides are structurally similar to the phenol extracted ones, with similar electrophoretic patterns and endotoxicity. The primary differences are in the amounts of nucleic acid and protein contaminants remaining after extraction. The TCA extracts contain ~2% RNA and ~10% denatured proteins, while phenol extracts contain up to 60% RNA and <1% protein. Subsequent purification by gel filtration chromatography removes much of protein present in the phenol-extracted LPS, but results in a preparation that contains 10-20% nucleic acids. Further purification using ion exchange chromatography yields an lipopolysaccharide product which contains <1% protein and <1% RNA.