Bacterial Endotoxin

A substance is used to activate the hypothalamic thermoregulatory centre by increasing its set point in order to induce the fever such substances are known as pyrogen. These substances stimulate the cold sensing neurons by inhibiting the heat-producing neurons as a result of this the body temperature gets increased more than the normal range by the stimulation of the hypothalamus, which causes fever. When the temperature of the body is increased, then the microbes present in the body may get killed or the growth of these microbes can increase so that is not able to tolerate the higher temperature conditions. Thus the rise in the body temperature increases the production of the antibodies, stimulates activity and motion, multiplies the number of white blood cells, etc.

The Natural Pyrogens are of Two Types:

  1. Endogenous pyrogens which is the host of the cytokines of the pyrogen.

  2. Exogenous pyrogens are a microbial substance. These include lipopolysaccharides. 

The liposaccharides are also called endotoxins. The endotoxin meaning is as follows, these are the large molecules that consist of polysaccharides and lipids that are composed of antigen O. The inner and outer core is joined by the covalent bonds. These endotoxins are found in the outer membrane of gram-negative bacteria. 

LPS toxin activity was first discovered by the Physician Richard Friedrich Johannes Pfeiffer, he is the one who termed the word endotoxin and also found the difference between the endotoxin and exotoxin. He considered it as a toxin that is present in the bacteria and is released to the outer environment only when the cell wall of the bacteria is ruptured. But according to some of the studies it is not required to destroy the cell wall of the bacteria in order to release the toxin. Rather the LPS toxin is secreted as a part of the physiological activity by the membrane vesicles. It is secreted in the form of OMVs (Outer Membrane Vesicle) of the bacteria; it also consists of proteins and other virulent substances.


LPS Endotoxin Composition


LPS Consists of Three Parts that include:

  1. O-Antigen: LPS consists of a repetitive glycan polymer that is known as O-antigen or O-polysaccharide or O-side chain present in the bacteria. This antigen is attached to the core of the oligosaccharide and comprises the outermost domain of the endotoxin molecule. The composition of the chain may vary from strain to strain. LPS is differentiated as rough and smooth by the presence or absence of the O chains. If the length of the O chain is full then it is considered as smooth LPS and if the length of the O chain is reduced or if it is absent then it is known as rough LPS. The cell membrane of the rough LPS is more capable of penetrating the hydrophobic antibiotics. 

  2. Core Oligosaccharide: The oligosaccharide compound is found present in the core domain and it is attached directly to lipid A. It also consists of sugars such as heptose and KDO (keto-deoxy octulosonic). Many of the bacteria also contain non-carbohydrate components such as amino acids, phosphate. 

  3. Lipid A: In normal circumstances, the phosphorylated glucosamine disaccharide is coated with fatty acids in order to form lipid A. The LPS is chained to the bacterial membrane with the help of hydrophobic fatty acid and the remaining LPS is projected from the surface of the cell. It is the main domain that is responsible for the toxicity of the bacterial endotoxin. 


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Functioning of Bacterial Endotoxin

We have seen the endotoxin meaning, now let us see its functions:

  • LPS is the type of glycolipids that are found present on the outer membrane of the bacterias such as Gram-negative bacteria. It plays an important role in the functionality and integrity of the outer membrane. These are capable of acting as immunomodulators and immunostimulation. 

  • LPS that is present on the cell membrane aims at increasing the negative charges thus stabilizes the overall membrane structure. 

  • It induces the strong from the immune system of the animal.

  • It acts as a myocardial depressant factor.

  • It involves the activation of the coagulation system.

  • A high conserved host enzyme is capable of detoxifying the LPS and it also converts the LPS that is present inside the intestine as an LPS inhibitor. 


Biological Effects on Hosts Due to Gram-Negative Bacteria

1. Immune Response: Lps can bind to the MD2 or TLR4 or CD14 receptors thus they act as prototypical endotoxin. These receptors are present in many cells including dendritic cells, macrophages, B-cells, and monocytes. These are involved in the secretion of nitric oxide, pro-inflammatory cytokines, and eicosanoids. In various cell types, LPS induces superoxide which is produced as a part of the cellular stress response, it is one of the major reactive oxygen species and expresses the toll-like receptor (TLR). LPS is an exogenous pyrogen. 

Since LPS plays an important role in the activation of the transcription factor it is under experimental research. It also produces many types of receptors that act as mediators and are involved in septic shock. Compared to other animals humans are more sensitive to LPS. 

2. Effect of Variability on Immune Response: Most valuable portion of the LPS molecule is the O-antigen. Whereas lipid A is the most conserved part but it can vary in composition. The gene clusters of LPS are highly variable in the terms of subspecies, strains, and species of bacterial pathogens of the animals and plants. 

3. Non-Colonial Pathways of LPS Recognition: Along with the TLR mediated pathways some of the members of the family of transient receptor potential ion channels are capable of recognizing the LPS. 


Delta Endotoxin

Delta endotoxins are produced by Bacillus thuringiensis and these are the pore-forming toxins. These are the primary toxins that are produced by the Bt crop and are useful for their insecticidal action. The bacteria produce crystals of the proteins during the process of spore formation hence they are also known as parasporal bodies. 

The Activated Region Consists of Three Domains, They are:

  • N-Terminal Domain: It is a helical bundle domain that is involved in the formation of pores and membrane insertion. 

  • Beta-Sheet Domain: It is the central domain that is involved in the binding of the receptor. 

  • C-Terminal Domain: It is known as the beta-sandwich domain and it is involved in the formation of the channel by interacting with the N-terminal.

When it is injected by the insects these get activated with the help of proteolytic cleavage. Where the C-terminal gets cleaved in some of the members whereas the N-terminal is cleaved in all the members. When it is activated it gets bound to the gut of the epithelium by causing cell lysis that results in death. 


LAL Endotoxin

LAL is expanded as a Limulus amebocyte lysate. DUring gram-negative sepsis, the endotoxins stimulate the host macrophages in order to release the inflammatory cytokines. But the excessive inflammation can cause the failure of the multiple organs that leads to death. Approximately 30 years since the LAL test is used to detect the endotoxin in the assurance of quality for the injectable drugs and medical devices. 

There are Three Basic Methods in the Test, They are:

  1. Gel-clot

  2. Turbidimetric

  3. Chromogenic

The main application of this test is in medical devices that are in contact with the blood. The pretreating of the samples to overcome the inhibition is the most time-consuming aspect of the testing of endotoxin. If the tested product causes less recovery of the endotoxin than expected then it is inhibitory to the test. If a high recovery is found then it causes enhancement. In order to get validated by the FDA, the inhibition and enhancement properties have to be overcome in order to release the final testing of the medical devices and the injectables. 


Conclusion

We got to know that these are found on the membrane of the cell. In the process of inflammation, endotoxins play an important role. With the severity of meningococcal disease and several other forms of sepsis the level of endotoxin can be correlated directly. A large number of studies in occupational epidemiology have shown that exposure to endotoxins increases the likelihood of organic dust toxic syndrome, chronic bronchitis, and asthma‐like syndrome.

FAQs (Frequently Asked Questions)

1. What are Endotoxins? Mention Some of the Examples of Bacterial Endotoxins.

Ans: The endotoxins are the large molecules that are found on the membrane of the gram-negative bacteria and these are made up of lipids and polysaccharides. 

Examples: Salmonella, Pseudomonas, Escherichia coli, etc. 

2. Endotoxins are More Harmful. Why?

Ans: Endotoxins are lipopolysaccharides that are found on the cell wall of Gram-negative bacteria, which can induce inflammation and cause fever as an immune response in higher organisms. Reaction to these bacterial endotoxins can lead to anaphylactic shock and death of patients.