1、 Protective effect of lipopolysaccharide on bacteria

The permeability of the normal outer membrane to hydrophobic molecules is extremely low, which protects the stability of the bacterial internal environment. If the bacteria are treated with EDTA, the lateral action of lipopolysaccharide molecules in the outer membrane is weakened due to the chelation of calcium ions by EDTA, and the lipopolysaccharide molecules are released from the cell wall, which makes the peptidoglycan layer in the cell wall more susceptible to lysozyme digestion and decomposition, At the same time, the permeability of the outer membrane to many hydrophobic reagents such as dyes and detergents is increased. Rough-type strains Rd1, Rd2, and Re have increased permeability to hydrophobic molecules, which also confirmed that lipopolysaccharide molecules play a protective role in the outer membrane. At present, it is believed that LPS is the pattern recognition molecule of gram-negative bacteria, and this structure is an indispensable molecule for its survival. If this structure is recognized by the host's receptors, such as CD14, TLR4, and other natural immune reaction molecules, resulting in the removal of GNB by the host, this mechanism is considered to be the result of evolution. GNB can undergo some changes, and the host's receptor can also undergo corresponding changes, but these changes are in the recognition of its common structure so that it cannot escape the responsibilities of the host, that is, the expression of the "arms race" between the microorganism and the host.

At present, the core glycolipid part, namely the KDO lipid A deficient strain, has not been isolated, indicating the importance of lipid AKDO in bacterial growth and reproduction, and other life activities.

The O-antigen-specific polysaccharide chain is hydrophilic and negatively charged, which can protect bacteria from the regulatory phagocytosis of phagocytes. The highly variable O-antigen-specific polysaccharide chain can protect bacteria from the clearance reaction of existing antibodies and digestive enzymes in the host. Strains with smooth lipopolysaccharide (that is, lipopolysaccharide molecules with complete or long O-antigen polysaccharide chain) have antiserum killing effect because of the steric blocking effect of long O-antigen polysaccharide chain, which prevents the complement complex from attacking and adhering to the hydrophobic outer membrane of the cell wall, thus avoiding the damage of compliment.

2、 Enhance non-specific immunity of the host

A low dose of endotoxin can activate B lymphocytes to produce polyclonal antibodies; Promote the development and maturation of T lymphocytes; Activate NK cell activity; Activate macrophages, enhance their phagocytosis and digestion ability, synthesize and secrete cytokines such as interferon, tumor necrosis factor, colony-stimulating factor, interleukin, and regulate immune response; In addition, endotoxin can activate complement through alternative ways and exert a series of biological effects of complement activation. Therefore, endotoxin can enhance the nonspecific immune ability of the body, resist radiation damage, promote the proliferation of granulocyte and monocyte, enhance the immune function of phagocytes, induce tumor necrosis, and enhance the ability to resist tumors through the above mechanisms, and have immune adjuvant activity, which is beneficial to the host body.

After the injection of endotoxin, the volume and weight of the liver and spleen increased, and the mononuclear-phagocytic system tissues such as the liver, lung, and lymph nodes proliferated, and cell division increased.

3、 Can induce endotoxin tolerance

Low-dose endotoxin can induce endotoxin tolerance, which is mainly the result of changes in the expression and properties of receptors, such as down-regulation of TLR4 expression; And changes in the activity of transcription regulatory factors, such as nuclear factor NF- κ In the composition of B complex, p65 increased and p50 decreased, which reduced the proportion of p65p50/p50p50, affected the translocation of nuclear factors into the nucleus and reduced the expression of its governing genes; Moreover, the expression of IRAK (Interleukin-1 receptor-associated kinase) decreased, and it did not approach and bind to the DD (death domain) of TLR4 cytoplasm, so it could not activate the downstream molecules of endotoxin signal transduction, which would inevitably affect the activation of transcription factors, and could not make TNF- α， IL-1 expression.

4、 Induce non-specific infection tolerance

Low-dose endotoxin activates immune cells, in which NF- κ B, AP-1, STAT, and other signal pathways are shared by many cytokines or in NF- κ B, etc. NF- κ Transcription factors such as B complex translocate into the nucleus from the cytoplasm to induce gene expression under its jurisdiction, affect signal transduction of other cytokines, pathogens, and products, and produce non-specific infection tolerance phenomenon through the expression of inhibitory cytokines, such as IL-4, IL-10, etc.

5、 Induce tumor necrosis and apoptosis