Many microorganisms produce toxins which play an important role in pathogenicity. Toxins are categorized into two: endotoxins and exotoxins. In this article, we are going to discuss the two types of toxins, there similarities, and differences.
Image 1: Endotoxins and exotoxins as illustrated in the images above.
Picture Source: differencebetween.net
Image 2: The mechanism of actions of endotoxins and exotoxins.
Picture Source: stackpathdns.com
What are Exotoxins?
They are a heat labile protein secreted by some species of bacteria. Exotoxins diffuse into the surrounding medium and cause damage to the host by disrupting the normal functions of the cell or by directly destroying the cells.
Exotoxins are extremely dangerous to the extent that they can cause severe damage to the host. (1, 2, 3, and 4)
There are many types of exotoxins but the most common ones are:
- Botulinum Toxin
- Cholera Toxin
- Diphtheria Toxin
Image 4: Endotoxins and their effect to the host.
Picture Source: tfd.com
What are Endotoxins?
They are heat stable proteins (lipopolysaccharide) that form structural components of the wall of gram-negative bacteria. They are liberated on the cell’s lysis or when the microorganisms die. Endotoxins are pyrogens; which are fever-causing agents.
Endotoxins are categorized as LPS or lipopolysaccharides. They contain fatty acids and disaccharide phosphates. They also contain O antigen and core polysaccharides. Their endotoxin activity is caused by the lipid A portion of lipopolysaccharides.
The lipid A component does not cause any harm to the tissues. However, the body’s immune system sees it as an indicator that there is a presence of harmful microorganisms. So, the body’s immune system will do its best to get rid of intruders. This is the immune system’s way of protecting the body. (3, 4, 5, and 6)
Endotoxin and Exotoxin similarities – “They both elicit an immune response.”
Differences between Endotoxins and Exotoxins
Endotoxins and exotoxins; although similarly elicit immune response do have a lot of differences. Their differences are shows in the table below.
|Basis of comparison||Endotoxins||Exotoxins|
|Chemical nature||Lipopolysaccharides (LPS) of gram negative bacteria; two components with distinct chemical and physical characteristics||Secreted by bacteria and primarily composed of proteins.|
|Locations||It is inside the cell of gram negative bacteria and will only be released after lysis.||Both gram negative and gram positive bacteria secrete exotoxins outside the cell.|
|Mode of action||Its mechanism of action includes TNF and Interlukin-1||It has various modes of actions.|
|Heat stability||They have a high tolerance to heat. In fact, they are stable at 250 degree Celsius for an hour.||They are heat liable. They can be destroyed once exposed at 600 to 800 degree Celsius of heat. They are unstable with the exemption of Staphylococcal enterotoxin. (6, 7)|
|Test used to detect the toxins||Endotoxins are detected by the limulus lysate assay test.||Exotoxins are detected using the following methods:|
|Immunogenicity||They have weak immunogenicity because they are not capable of producing antitoxins. (4, 8)||They are immunogenic. They trigger the production of antibodies which will target the effect of toxins. With the help of the body’s immune system, exotoxin will secrete antitoxin which aids in neutralizing toxins. (8, 9)|
|Vaccines||There are no vaccines available as toxoids cannot be made.||Vaccines are readily available as toxoids can be created by treating formaldehyde.|
|Diseases linked with||Typhoid fever||Scarlet fever|
|Urinary tract infections||Diphtheria|
|Coronary artery disease||Gas gangrene|
|Neonatal necrotizing enterocolitis||Scalded skin syndrome|
|Ulcerative colitis||Antibiotic-associated diarrhea|
|Sepsis by gram negative rods|
|Can cause fever||Yes||No|
|Enzyme activity||It does not have any enzymatic activity.||Most of the activities are enzymatic in nature.|
|Molecular weight||50 to 1000 KDa||10 KDa|
|Denaturing||It cannot be denatured on boiling.||It gets denatured on boiling|
|Specificity||It is non-specific.||It is specific to a particular strain of bacteria.|
|Toxicity||It is moderately toxic.||It is highly toxic.|
|Examples||E. coli||S. aureus|
|Salmonella typhi||Bacillus cereus|
|Streptococcus pyogenes (4, 7, 9, and 10)|
Summary of the clinical features and significance
- They are LPS (lipopolysaccharides).
- They are attached to the bacterial cell wall.
- For endotoxins to be releases, the cell needs to undergo lysis.
- They are mostly produced by Gram-negative bacteria.
- They are heat stable.
- Production of endotoxins is dependent on the genes located on the bacterial chromosome.
- They remain active even at 100 degree Celsius.
- They have a non-specific effect on the body.
- There is no specific receptor for the entry of endotoxins.
- They can only be lethal when present in large quantities.
- They cannot be neutralized by antibodies.
- They can produce fever to the host.
- They have a poor antigenic nature. (3, 5, 7, and 9)
- They are proteins.
- Exotoxins are released even without requiring the cells to undergo lysis.
- They are usually produced by gram positive bacteria.
- They are secreted into the external medium by the bacterial cells.
- They are heat labile. They can get destroyed when exposed to heat with a temperature beyond 600 degree Celsius.
- Their effects are specific.
- They use specific receptors to enter into the host cell.
- Even in a small amount, they can be extremely lethal.
- They have a high antigenic nature.
- They can be neutralized by antibodies.
- Vaccines are available for exotoxins.
- They can be converted to toxoids.
- They rarely cause fever. (2, 4, 9, and 10)
Toxins are chemical substances produced by microorganisms to harm the host and cause diseases. There are two types of toxins; endotoxins and exotoxins. Toxigenicity is the ability of the organisms to produce toxins.
The differences between endotoxins and exotoxins are discussed above. Exotoxins are lethal even in small quantities. Fortunately, there are vaccines available to fight them. On the other hand, endotoxins are less lethal but can cause fever to the host.
Exotoxins are secreted by bacteria and release outside the cell whereas endotoxins are bacterial toxins located within the cells.