The large molecules formed from polysaccharides and lipids are known as lipopolysaccharides. The O-polysaccharide, lipid A, and core are the components of LPS. They are present in the outer membranes of bacteria and function as endotoxins. Eliciting immune responses in animals is also one of its important functions. The negative charge of cell membranes is increased by LPS, which helps in stabilizing the membrane structure. Its layer in Gram negative bacteria is useful in maintaining structural integrity.
The structure of lipopolysaccharide is formed of three components.
O-polysaccharide: An O-polysaccharide or O-antigen is formed of a repetitive glycan polymer. It is also known as O side chain. The outer membranes of an LPS are formed of O-antigens. These are connected to the core oligosaccharide. Composition of the chain differs in different types of bacteria. The E. coli alone has strains that have 160 different compositions of O-antigens.
Core: The core of LPS is formed of oligosaccharide and sugars such as 3-deoxy-D-manno octulosonic acid and heptose. The oligosaccharide at the core is attached with lipid A. Apart from carbohydrates, the core also contains amino acids, phosphate and ethanolamine substituents.
Lipid A: The lipid A component is formed of phosphorylated glucosamine disaccharide, which is surrounded by multiple fatty acids. These fatty acid chains of lipid A help in anchoring LPS to the membrane of bacteria. Toxicity of Gram-negative bacteria is mostly attributed to their lipid A component. When lysis, i.e., breakdown of bacterial cells takes place due to the effect of the immune system, lipid A components circulated in the body results in diarrhea, fever, and even endotoxic shocks which could be fatal.
The lipid A present in bacteria acts as endotoxins. They are released in the lysis of bacteria. They are responsible for causing diseases, and are found in the outer layers of Gram-negative bacteria. The term lipopolysaccharide is used in exchange of endotoxin. The average size of endotoxins is 10 kDa (Kilo dalton). However, they could be as big as 1000 kDa. Human beings are able to produce antibodies against endotoxins to save themselves from the possible ill-effects. There are different types of endotoxins and they are large in numbers. Therefore, the antibodies may not prove to be effective against them. Even a small amount of endotoxin is enough to lower the blood pressure of the body, induce fever, and cause inflammation & coagulation.
The cells of Escherichia coli (E. coli) are enveloped by an asymmetric bilayer formed of phospholipid, LPS, and outer membrane proteins (OMPs). A sigma (E)- dependent response is triggered if the integrity of the outer membrane is affected due to chemical and physical stress.
LPS is an important component of bacteria and form the cell walls. It isn't only significant in stabilizing the overall membrane structure, but also in protecting the bacteria against certain chemicals.