Lipopolysaccharide
Lipopolysaccharides (LPS) are glycolipids present in the membrane of most Gram-Negative bacteria. LPSs are large molecules consisting of a hidrophobic portion (Lipid-A), wich anchors the LPS to the outer-leaflet of the external membrane, an oligosaccharide core, and a polysaccharide (O-antigen) exposed to the outside of the cell. LPSs are also know as endotoxins and elicit strong immune responses in animals. For plant-colonizing bacteria, LPS play an important role in the molecular communication between plant-microbe interaction.
LPSs comprise three parts:
1. O-antigen (or O-polysaccharide) 2. Core oligosaccharide 3. Lipid A
Lipid A
Lipid A is normally a phosphorylated glucosamine disaccharide decorated with multiple fatty acids. These hydrophobic fatty acid chains anchor the LPS into the bacterial membrane and the rest of the LPS projects from the cell surface. The lipid A domain is responsible for much of the toxicity of Gram-negative bacteria. When bacterial cells are lysed by the immune system, fragments of membrane containing lipid A are released into the circulation, causing fever, diarrhea, and possible fatal endotoxic shock (also called septic shock).
Core oligosaccharide
The Core oligosaccharide attaches directly to lipid A and normally contains sugars such as heptose and 3-deoxy-D-mannooctulosonic Acid (also known as KDO, keto-deoxyoctulosonate).
O-antigen
When LPS contains a repetitive glycan polymer this is referred to as the O antigen, O polysaccharide, or O chain of the bacteria. O antigen is attached to the core oligosaccharide, and comprises the outermost domain of the LPS molecule. The composition of the O chain varies from strain to strain, for example there are over 160 different O antigen structures produced by different E. coli strains. The presence or absence of O chains determine whether the LPS is considered rough or smooth. Full length O-chains would render the LPS smooth while the absence or reduction of O-chains would make the LPS rough. Bacteria with rough LPS usually have more penetrable cell membranes to hydrophobic antibiotics since a rough LPS is more hydrophobic. O antigen is exposed on the very outer surface of the bacterial cell, and as a consequence, is a target for recognition by host antibodies.
LPS modifications
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. 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.