<P> A repetitive glycan polymer contained within an LPS is referred to as the O antigen, O polysaccharide, or O side - chain of the bacteria . The 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 determines whether the LPS is considered rough or smooth . Full - length O - chains would render the LPS smooth, whereas 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 . </P> <P> The Core domain always contains an oligosaccharide component that attaches directly to lipid A and commonly contains sugars such as heptose and 3 - Deoxy - D - manno - oct - 2 - ulosonic acid (also known as KDO, keto - deoxyoctulosonate). The LPS Cores of many bacteria also contain non-carbohydrate components, such as phosphate, amino acids, and ethanolamine substituents . </P> <P> Lipid A is, in normal circumstances, 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). The Lipid A moiety is a very conserved component of the LPS . </P> <P> Lipooligosaccharides (LOS) are glycolipids found in the outer membrane of some types of Gram - negative bacteria, such as Neisseria spp. and Haemophilus spp . The term is synonymous with the low molecular weight form of bacterial LPS . LOS plays a central role in maintaining the integrity and functionality of the outer membrane of the Gram negative cell envelope . Lipooligosaccharides play an important role in the pathogenesis of certain bacterial infections because they are capable of acting as immunostimulators and immunomodulators . Furthermore, LOS molecules are responsible for the ability of some bacterial strains to display molecular mimicry and antigenic diversity, aiding in the evasion of host immune defenses and thus contributing to the virulence of these bacterial strains . </P>

The toxic lipopolysaccharides produced by most gram-negative bacteria are called exotoxins