In this article, you can learn about different types of exotoxins and list of exotoxins. Exotoxins are proteins produced by both gram-positive bacteria and gram-negative bacteria. There are different types of exotoxins. They are considered to be among the most lethal substances as they can easily disseminate in blood for their high solubility. A little amount of bacterial exotoxins is sufficient to be a lethal dose. For example, 1mg of botulinum toxins is enough to kill 1 million guinea pigs.
Bacterial exotoxins stimulate the production of antitoxins as well as potential immune response. Treatment of exotoxins with heat or formaldehyde or Iodine or other chemicals makes it loose of pathogenicity but retains its antigenicity. So it can be used as a toxoid or vaccines. In this context, three types of exotoxins are discussed with their target cells, mechanism of actions and diseases caused by them.
How Many Types of Exotoxins are there?
There are three types of exotoxins. They are:
1. A-B toxin:
A-B toxins are the polypeptides composed of part A and part B. Part A is an active enzymatic component taking part in altering the functions of host cells through inhibition of protein synthesis. Part B is a binding component that mediates the attachment of the toxin to the receptors of host cells.
Bacteria that produce exotoxins: Clostridium tetani, Clostridium diphtheria, Vibrio cholerae, S. aureus.
Toxins: Tetanospasmin, Diphtheria toxin, Cholera toxin, Exfoliatin.
Target cells: Different types of cells throughout the body.
Mechanisms of these types of exotoxins: Different toxins show different mechanisms of actions. Few of them are as below:
- Tetanospasmin inhibits the release of inhibitory neurotransmitters.
- Diphtheria toxins cause ADP ribosylation of EF-2 resulting in inhibition of protein synthesis.
- Cholera toxins cause ADP ribosylation of adenylate cyclase, leading to rising in cyclic AMP.
- Neurotoxin prevents the transmission of nerve impulses: flaccid paralysis results.
- Neurotoxin blocks nerve impulses to muscle relaxation pathways: results in uncontrollable muscle contractions.
- Cytotoxin inhibits protein synthesis, especially in nerve, heart, and kidney cells.
- Enterotoxin causes secretion of large amounts of fluids and electrolytes that result in diarrhea.
- Exfoliatin is a protease that breaks down desmoglein protein in desmosomes of skins causing scalded skin syndromes.
Diseases: Spastic paralysis Myopathy, Polyneuropathy, Profuse watery diarrhea
Botulism, Tetanus, Diphtheria, Scalded skin syndrome, Cholera, Traveler’s diarrhea, Anthrax.
2. Membrane-disrupting Toxins:
Among three types of exotoxins, membrane-disrupting toxins are the toxins that cause damage to host cells by breaking down the structure of their plasma membranes or by forming protein channels or by disrupting the phospholipids.
Bacteria that produce exotoxins: Staphylococcus aureus, Listeria monocytogenes, Clostridium perfringens, Clostridium difficile.
Toxins: Leukocidin, α-Toxin (lecithinase), Hemolysin, Listeriolysin, Perfringolysin-O, Streptolysins. Streptolysin O and Streptolysin L.
Target cells: Many cell types including erythrocytes, phagocytic leukocytes, and macrophages.
Mechanisms: Different toxins show different mechanisms of actions. Few of them are as below:
- Leukocidins and Hemolysins destroy their target cells (leukocytes and erythrocytes respectively) by forming protein channels in the plasma membrane of their target cells.
- The alpha-toxin such as lecithinase causes lysis of cell membranes by breaking down lecithin into phosphorylcholine and diglyceride.
- Phospholipase removes polar head groups from phospholipids.
- Cytotoxin destroys massive red blood cell (hemolysis) and destroys host cell cytoskeleton.
- Enterotoxin causes the release of fluids and electrolytes resulting in diarrhea.
Diseases: Tissue necrosis, Gas gangrene, Food poisoning, Antibiotic-associated diarrhea.
Superantigens are the antigens that stimulate the production of a large amount of T cells and also stimulate these T cells to produce a large amount of cytokine.
Bacteria that produce exotoxins (superantigens): Streptococcus pyogenes, Staphylococcus aureus.
Superantigens/toxins: Streptococcal pyogenic exotoxin, Toxic shock syndrome toxin, Staphylococcal enterotoxin A (SEA).
Target cells: T cells, macrophages.
Mechanisms: Different toxins/superantigens show different mechanisms of actions. Few of them are as below:
- Both streptococcal pyogenic exotoxin and toxic shock syndrome toxin cause T cell stimulation, the release of cytokines (IL-1, IL-2, TNF), possible enhancement of LPS activities.
- Released cytokines stimulate enteric nervous systems to activate the vomiting center in the brain.
- Enterotoxin causes secretion of fluids and electrolytes that results in diarrhea.
- The toxin causes secretion of fluids and electrolytes from capillaries that decreases blood volume and lowers blood pressure.
Diseases: Fever, rash, nausea, vomiting, diarrhea toxic shock-like syndrome, Toxic shock syndrome, food poisoning.
List of Exotoxins:
This list of exotoxins includes 10 exotoxins that are produced by several strains of gram-positive bacteria.
1. Diphtheria toxin:
A bacteria called Corynebacterium diphtheriae produces this toxin. This toxin causes diphtheria by inhibiting protein synthesis and thus leads to the death of the cells. This cell death results in diphtheria that is characterized by the formation of pseudomembrane in the throat and myocarditis.
2. Tetanus toxin:
Tetanus toxin (tetanospasmin) is a neurotoxin that blocks the production of the inhibitory neurotransmitter glycine. This toxin causes muscle spasms and spastic paralysis. Clostridium tetani produce this toxin.
3. Botulinum toxin:
Botulinum toxin is a neurotoxin that is considered to be one of the most toxic compounds. Only 1 microgram is enough to cause death to a human. Clostridium botulinum produces this toxin that causes flaccid paralysis.
4. Clostridial Exotoxin (Exotoxin A and Exotoxin B):
Clostridium difficile produces two exotoxins. They are Exotoxin A and Exotoxin B that leads to watery diarrhea and form pseudomembrane. Both Exotoxin A and Exotoxin B cause pseudomembranous colitis. Both of these toxins hampers cytoskeletal integrity, cause apoptosis, and death of the intestinal cells.
5. Anthrax toxin:
Anthrax toxin includes three exotoxins such as edema factor, lethal factor, and protective antigen. These toxins are produced by Bacillus anthracis that causes Anthrax.
6. Toxic shock syndrome toxin (TSST):
TSST is a superantigen that causes toxic shock syndrome or fever. This toxin stimulates the release of a higher amount of cytokine in the bloodstream. These cytokines then cause toxic shock syndrome. Several strains of gram-positive bacteria such as S. aureus and S. pyogenes produces this toxin.
7. Staphylococcal enterotoxin:
Staphylococcus aureus is a foodborne bacteria that produce staphylococcal enterotoxin and causes food poisoning.
This toxin is also a superantigen that stimulates the release of a higher amount of cytokine and stimulates the vomiting center in the brain. Thus it causes nausea, vomiting, and diarrhea.
Staphylococcus aureus produces another exotoxin called exfoliatin. It causes scalded skin syndrome by damaging desmoglein protein of the desmosome of the skin.
9. Panton-Valentine (PV) leukocidin:
Methicillin-resistant Staphylococcus aureus produces this toxin. This toxin damages various cells such as white blood cells or leukocytes. This toxin is also produced by Streptococcus pyogenes. This toxin causes necrotic lesions and hemorrhagic pneumonia.
10. Erythrogenic toxin:
Streptococcus Pyogenes produces this toxin. This is a cytotoxin that includes three variations such as erythrogenic toxin A, erythrogenic toxin B, erythrogenic toxin C. This toxin causes red skin rash by destroying plasma membranes of blood capillaries under the skin. Red skin rash is a characteristic symptom of scarlet fever.
READ MORE: What is Exotoxin