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toxin, poison produced by living organisms. Toxins are classified as either exotoxins or endotoxins. Exotoxins are a diverse group of soluble proteins released into the surrounding tissue by living bacterial cells. Exotoxins have specific reaction sites in the host; e.g., tetanus and botulinum exotoxins affect nerve tissue, and streptococcal toxins attack vascular tissue. Plants and animals also produce protein toxins. Some, such as cobra venom, are enzymes that destroy substances in host tissue. Endotoxins are polysaccharide and phospholipid substances found in the cell walls of bacteria that are freed when the cells die and break up. The pathologic effects of endotoxins, similar for all bacterial sources, include fever, shock, and intestinal hemorrhage. In sufficiently low doses toxins stimulate the production of antibodies, or antitoxins, in the host, and toxins of a specific bacterial species have been injected to elicit formation of antibodies against the disease caused by the bacteria. Toxoids are protein toxins that have been heated or chemically treated to deprive them of their toxicity but not of the ability to induce the formation of antibodies. See venom.
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Properly, a poisonous protein, especially of bacterial origin. However, nonproteinaceous poisons, such as fungal aflatoxins and plant alkaloids, are often called toxins. See Aflatoxin, Alkaloid

Bacterial exotoxins are proteins of disease-causing bacteria that are usually secreted and have deleterious effects. Several hundred are known. In some extreme cases a single toxin accounts for the principal symptoms of a disease, such as diphtheria, tetanus, and cholera. Bacteria that cause local infections with pus often produce many toxins that affect the tissues around the infection site or are distributed to remote organs by the blood. See Cholera, Diphtheria, Staphylococcus

Toxins may assist the parent bacteria to combat host defense systems, to increase the supply of certain nutrients such as iron, to invade cells or tissues, or to spread between hosts. Sometimes the damage suffered by the host organism has no obvious benefit to the bacteria. For example, botulinal neurotoxin in spoiled food may kill the person or animal that eats it long after the parent bacteria have died. In such situations it is assumed that the bacteria benefit from the toxin in some other habitat and that the damage to vertebrates is accidental. See Food poisoning

Certain bacterial and plant toxins have the unusual ability to catalyze chemical reactions inside animal cells. Such toxins are always composed of two functionally distinct parts termed A and B, and they are often called A-B toxins. The B part binds to receptor molecules on the animal cell surface and positions the toxin upon the cell membrane. Subsequently, the enzymically active A portion of the toxin crosses the animal cell membrane and catalyzes some intracellular chemical reaction that disrupts the cell physiology or causes cell death. See Immunologic cytotoxicity

A large group of toxins breach the normal barrier to free movement of molecules across cell membranes. In sufficient concentration such cytolytic toxins cause cytolysis, a process by which soluble molecules leak out of cells, but in lower concentration they may cause less obvious damage to the cell's plasma membrane or to its internal membranes. See Cell membranes, Cell permeability

Tetanus and botulinal neurotoxins block the transmission of nerve impulses across synapses. Tetanus toxin blockage results in spastic paralysis, in which opposing muscles contract simultaneously. The botulinal neurotoxins principally paralyze neuromuscular junctions and cause flaccid paralysis.

Gram-negative bacteria, such as Salmonella and Hemophilus, have a toxic component in their cell walls known as endotoxin or lipopolysaccharide. Among other detrimental effects, endotoxins cause white blood cells to produce interleukin-1, a hormone responsible for fever, malaise, headache, muscle aches, and other nonspecific consequences of infection. The exotoxins of toxic shock syndrome and of scarlet fever induce interleukin-1 and also tumor necrosis factor, which has similar effects. See Endotoxin, Scarlet fever, Toxic shock syndrome

Toxoids are toxins that have been exposed to formaldehyde or other chemicals that destroy their toxicities without impairing immunogenicity. When injected into humans, toxoids elicit specific antibodies known as antitoxins that neutralize circulating toxins. Such immunization (vaccination) is very effective for systemic toxinoses, such as diphtheria and tetanus. See Antibody, Immunity, Vaccination

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a substance of bacterial, vegetable, or animal origin capable of depressing physiological functions in such a way as to cause sickness or death in animals and humans. Chemically, toxins are either proteins or polypeptides. In contrast to other organic and inorganic poisonous substances, toxins, upon entering an organism, cause the formation of antibodies. (The molecular weight of toxins exceeds 4,000–5,000; substances of lower molecular weight are not immunogenic.) Toxins are contained in the poisons produced by, for example, snakes, scorpions, spiders, and certain plants.

The most widely distributed and thoroughly studied toxins are the bacterial type, of which several hundred are known. Bacterial toxins are subdivided into exotoxins and endotoxins. Exotoxins, which are secreted into the environment as a result of the vital activities of bacteria, are specific in their effect on organisms. Examples include neurotoxins and cytotoxins. Certain microorganisms secrete highly potent toxins that cause botulism, tetanus, diphtheria, and food poisoning. Endotoxins, which are given off after the death of bacteria, represent such normal products of bacterial metabolism as enzymes. These toxins disrupt the metabolism of biogenic amines in animals and humans. They are not specific in their effect. (See Table 1 on page 268 for data on the most important toxins.)

Bacterial toxins were discovered in 1888 by the French scientist P. Roux and the Swiss scientist A. Yersin, who obtained the toxin of the bacillus causing diphtheria (Corynaebacterium diphtheriae). This discovery made possible the development of detoxification methods that did not involve the destruction of the microorganisms producing the toxins. A successful attempt to use antitoxins (antibodies) was made in 1890 by the German bacteriologist

Table 1. Important toxins
 SourceMolecular weightDose causing death in 50% of experimental animals
Botulinus toxin A. . . . . . . . . .Clostridium botulinum150,0002.6 × 10–81.7 × 10–13
Botulinus toxin B. . . . . . . . . .C. botulinum167,0001.0 × 10–80.6 × 10–13
Tetanus toxin. . . . . . . . . .C. tetani140,0002.8 × 10–82.0 × 10–13
Ricin. . . . . . . . . .Seeds of castor-oil plant65,0002.8 × 10–34.3 × 10–8
Taipan toxin. . . . . . . . . .Taipan venom42,0002.0 × 10–34.8 × 10–8
β-bungarotoxin. . . . . . . . . .Krait venom28,5002.5 × 10–28.8 × 10–7
Cobrotoxin. . . . . . . . . .Cobra venom6,7825.0 × 10–27.4 × 10–6
Toxin II. . . . . . . . . .Scorpion venom7,2490.9 × 10–21.2 × 10–6

E. von Behring, who established that the blood serum of animals immunized by sublethal doses of toxins possessed prophylactic and therapeutic properties. In 1924 the French scientist G. Ramon proposed a detoxification method that would preserve the immune properties of toxins; here, the toxins were treated with Formalin, as a result of which toxoids—nonpoisonous derivatives of toxins—were formed. When introduced into the organism, toxoids help to create immunity to the corresponding toxins. In the late 1950’s, with advances in protein chemistry and the development of methods for purifying and identifying proteins, it became possible not only to modify toxins selectively, but also to separate toxoids from unconverted toxins.

Toxins are also classified according to their effect on the organism. Neurotoxins act on various stages of nerve impulses. Thus, certain bacterial toxins interfere with the conductivity of nerve fibers. Taipan toxin and β-bungarotoxin act on the presynaptic membrane, suppressing the secretion of the mediator acetylcholine. Cobrotoxin and other toxins of this class (the amino acid sequence having been established for 30 of the several dozen known) block the acetylcholine receptor of the postsynaptic membrane. Cytotoxins, which are highly surface-active, destroy biological membranes. Such toxins are often encountered in snake venom; they are similar to snake neurotoxins in chemical structure but different with regard to functionally important amino acids. Cytotoxins may cause lysis of blood cells. Toxins acting as inhibitors suppress the activity of certain enzymes and thus disrupt metabolic processes. Toxins acting as enzymes (proteases, nucleases, hyaluronidases, phospholipases) destroy (hydrolyze) such important components of the organism as proteins, nucleic acids, polysaccharides, and lipids.

The use of toxins is limited to the production of toxoids. Neurotoxins are used as selectively acting agents in electrophysiologic and clinical research on the mechanics of stimulus transmission in the nervous system.

The term “toxin” is often incorrectly applied to natural non-protein substances that disrupt functions of an organism.


Toksiny-anatoksiny i antitoksicheskie syvorotki. Moscow, 1966.
lady pchel i zmei v biologii i meditsine: Sb. st. Gorky, 1967.
Venomous and Poisonous Animals and Noxious Plants of the Pacific Region. Oxford, 1963.
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Karlsson, E. “Chemistry of Some Potent Animal Toxins.” Experientia, 1973, vol. 29, no. 11, pp. 1319–27.
Zlotkin, F. “Chemistry of Animal Venoms.” Experientia, 1973, vol. 29, no. 12, pp. 1453–66.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


Any of various poisonous substances produced by certain plant and animal cells, including bacterial toxins, phytotoxins, and zootoxins.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


1. any of various poisonous substances produced by microorganisms that stimulate the production of neutralizing substances (antitoxins) in the body
2. any other poisonous substance of plant or animal origin
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
Acute diarrhea in horses of the Potomac River area: examination for clostridial toxins. Journal of the American Veterinary Medical Association, v.185, n.4, p.433-435, 1984.
Frequency of binary toxin genes among Clostridium difficile strains that do not produce large clostridial toxins. J Clin Microbiol 2003; 41 : 5227-32.
difficule large clostridial toxins, antibiotic resistance determinants in C.