What is Endotoxins?

Endotoxins are biological toxins associated with the outer cell wall membrane of gram-negative bacteria such as E.coli, Salmonella, Shigella, Pseudomonas, vibrio cholera, etc. (Endotoxins are invariably associated with gram-negative bacteria whether the organisms are pathogenic or not). Endotoxins cause powerful inflammatory reactions in humans which include fever, flu-like symptoms, cough, headache, and respiratory distress.

Lipopolysaccharides, also known as endotoxins, are large molecules consisting of a lipid and a polysaccharide composed of O-antigen, outer core, and inner core joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria.

Endotoxins are complex lipopolysaccharides (LPS). LPS has two components, Polysaccharide & lipid A. Toxicity is associated with the lipid component (Lipid A) and immunogenicity is associated with the polysaccharide components. The cell wall antigens (O antigens) of Gram-negative bacteria are components of LPS. LPS elicits a variety of inflammatory responses in animals. Because it activates complement by the alternative (properdin) pathway, it is often part of the pathology of Gram-negative bacterial infections.

Endotoxins are toxic to most mammals. Even though endotoxins are strong antigens, they seldom elicit immune responses which give full protection to the animal against secondary challenges with the endotoxin. They cannot be toxoided. Regardless of the bacterial source, all endotoxins produce the same range of biological effects in the animal host.

Compared to the classic exotoxins of bacteria, endotoxins are less potent and less specific in their action, since they do not act enzymatically. Endotoxins are heat stable (boiling for 30 minutes does not destabilize endotoxin), but certain powerful oxidizing agents such as superoxide, peroxide, and hypochlorite, degrade them. Endotoxins, although strongly antigenic, cannot be converted to toxoids.

What are endotoxin and exotoxin?

Exotoxins are usually heated labile proteins secreted by certain species of bacteria which diffuse into the surrounding medium. Endotoxins are heat-stable lipopolysaccharide-protein complexes that form structural components of the cell wall of Gram-Negative Bacteria and are liberated only on cell lysis or death of bacteria.

Differences Between Exotoxins and Endotoxins

Many bacteria produce toxins, enzymes, and pigments. Toxins and enzymes play an important role in pathogenicity.

Toxins are of two types:

1. Exotoxins are usually heated labile proteins secreted by certain species of bacteria which diffuse into the surrounding medium.
2. Endotoxins are heat-stable lipopolysaccharide-protein complexes that form structural components of the cell wall of Gram-Negative Bacteria and are liberated only on cell lysis or death of bacteria.

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Some of the differences between Exotoxins and Endotoxins are as follows:

Sr. No.	Exotoxins	Endotoxins
1	Excreted by organisms, living cell	Integral part of cell wall
2	Found in both Gram positive and Gram-Negative bacteria	Found mostly in Gram Negative Bacteria
3	It is polypeptide	It is lipopolysaccharide complex.
4	Relatively unstable, heat labile (60°C)	Relatively stable, heat tolerant
5	Highly antigenic	Weakly immunogenic
6	Toxoids can be made by treating with formalin	Toxoids cannot be made
7	Highly toxic, fatal in µg quantities	Moderately toxic
8	Usually binds to specific receptors	Specific receptors not found
9	Not pyrogenic usually, Toxin Specific	Fever by induction of interleukin 1 (IL-1) production, Shock
10	Located on extrachromosomal genes (e.g. plasmids)	Located on chromosomal genes
11	Filterable	Not so
12	It has mostly enzymatic activity	It has no enzymatic activity
13	Its molecular weight is 10KDa	Its molecular weight is 50-1000KDa
14	On boiling it gets denatured.	On boiling it cannot be denatured.
15	Detected by many tests (neutralization, precipitation, etc.)	Detected by Limulus lysate assay
16	Examples: Toxins produced by Staphylococcus aureus, Bacillus cereus, Streptococcus pyogenes, Bacillus anthracis (Alpha-toxin, also known as alpha-hemolysin (Hla))	Examples: Toxins produced by E. coli, Salmonella Typhi, Shigella, Vibrio cholera (Cholera toxin also known as choleragen)
17	Diseases: Tetanus, diphtheria, botulism	Diseases: Meningococcemia, sepsis by gram negative rods

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