Endotoxins vs Exotoxins
Microbial toxins are noxious substances produced by the microorganisms such as bacteria and fungi in order to promote their pathogenicity and disease-causing process. The microbial toxins usually destroy the host tissue and they facilitate the infection by disabling the host immune system. A disease that results from a specific toxin is denoted as ‘Intoxication’. A toxin is a substance that alters the normal metabolism of host cells with deleterious effects. The term ‘Toxemia’ refers to the condition caused by the toxins that have entered the bloodstream of the host.
The toxins produced by bacteria are categorized into two main categories: (1) Endotoxins and (2) Exotoxins.
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(1). Endotoxins: They are also called as Lipopolysaccharides or LPS. LPS are present on the outer membrane of the cell wall of Gram-negative bacteria that, under certain circumstances, become toxic to specific hosts. Lipopolysaccharides are called endotoxins because they are bound to the bacterium and they are released only when the bacterial cells lyse.
(2). Exotoxins: They are heat labile, proteinaceous toxic substances that are released by some bacteria into the surrounding. Most of the exotoxins are produced by Gram-positive bacteria. Some Gram-negative bacteria can also produce exotoxins. Exotoxins are among the most lethal and toxic substances known; they are toxic even in nanogram-per-kilogram concentrations. Deadly pathogenic conditions like Botulism, Diphtheria, Tetanus, Cholera, Anthrax, Pertussis etc. are caused by bacterial exotoxins.
The following table summarizes the Differences between Bacterial Endotoxins and Exotoxins.
Difference between Bacterial Endotoxin and Exotoxin
Sl. No. Endotoxin Exotoxin
1 Endotoxins are Lipopolysaccharides (LPS) Exotoxins are proteins
2 Endotoxins are not secreted by the bacterium, they remain attached to the bacterial cell Exotoxins are actively secreted into the external medium by the bacterial cells
3 Cell lysis is required for the release of endotoxins Cell lysis is not required for the release of exotoxins
4 Produced by Gram-negative bacteria Produced usually by Gram-positive bacteria. (some by Gram-negative bacteria)
5 The genes for the production of endotoxin are located on bacterial chromosome The genes for the production of exotoxins are located on plasmids of bacteriophage genome
6 Endotoxins are heat stable Exotoxins are heat labile
7 Endotoxins stay active even at 100oC Exotoxins usually destroyed beyond 60oC
8 Endotoxins mediate toxicity in the host through interleukins and Tumor Necrosis Factor Exotoxins mediate toxicity in the host by mostly enzyme-like mechanism.
9 The effects of endotoxins is non-specific The effects of exotoxins is specific
10 Endotoxins do not have any affinity towards specific tissues. Exotoxins shows affinity towards specific tissues of the host
11 Usually no specific receptors for the entry endotoxins. Exotoxins usually use specific receptors for entering into the host cell
12 Endotoxins are lethal only in large quantities Exotoxins are lethal in very minute quantities (lethal even in ng/kg)
13 Endotoxins are poorly antigenic in nature Exotoxins are highly antigenic in nature
14 Endotoxins cannot be neutralized by antibodies Exotoxins can be neutralized by antibodies
15 No effective vaccines are available against endotoxins Effective vaccines are available against exotoxins
16 Endotoxins are not converted to toxoids* Exotoxins are converted to toxoids
17 Produce fever in host Usually, do not produce fever
18 Diseases caused by endotoxin: Gram-negative bacterial sepsis, meningococcemia Disease caused by exotoxin: Botulism, Diphtheria, Tetanus, Cholera, Anthrax, Pertussis
*Toxoids: They are the chemically modified toxin from a pathogenic microorganism. Toxoids are no longer toxic but are still antigenic and thus they can be used as a vaccine.
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