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Veterinary bacteriology: information about important bacteria
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Bacterial toxins are important virulence factors and they are usually classified as exotoxins (exo = outside) or endotoxins (endo = inside). Exotoxins are actively synthesized and secreted, whereas endotoxins are part of the bacteria and are released upon bacterial death and lysis. Most endotoxins are localized to the bacterial envelope and often, only lipopolysaccharides (LPS) of the outer membrane of gram negative bacteria are affiliated to thid group (see also lipopolysaccharide above). Endotoxins are heat stable.


Exotoxins are typically heat sensitive (heat labile) proteins, but some are heat stable polypeptides. Exotoxins may be formed by both gram positive and gram negative bacteria. Among bacterial exotoxins, some of natures most potent toxins are found. The lethal dose for humans of e.g. the botulinum toxin is only 1-2 ng (1 ng = 0,000 000 001 g) per kg body weight at intraveneous administration. Thus, the botulinum toxin is approximately 6 000 000 times more toxic than the rattle snake venom.


Exotoxins can in turn be classified in various ways, e.g. according to:

  • The tissue they affect: e.g. enterotoxins, hepatotoxins, leukocidins and neurotoxins.
  • The bacterium that produces the toxin: e.g. anthrax toxin, botulinum toxin, cyanotoxin, shiga toxin, staphylococcal toxin and tetanus toxin.
  • The structure of the toxin: e.g. AB toxins, which consist of two different protein subunits, A and B. A and B refers to active and binding, respectively, i.e. the actions of the subunits on the target cell. There are also AB5-toxins, which consist of 5 identical subunits (B) and one unique subunit (A), which has a different structure.
  • The action of the toxin: e.g. invasins, hemolysins, toxins with enzymatic activity and channel forming toxins. Invasins make it easier for the bacteria to invade the tissue of the host animal and hemolysins can lyse the erythrocytes (and other cells) of the host animal (see hemolysis above). Toxins with enzymatic activity can be lipases, which split phospholipids in the cell membrane which in turn will cause the host cell to be fragmented (see also hemolysis above). Channel forming toxins (= porins) will get the ion gradient over the cell membrane to collapse and that will also cause fragmentation of the host cell (see also hemolysis above).
  • The target organelle within or outside the host cell may be e.g. extracellular matrix, the plasma membrane, the cytoplasm and the immune system. Toxins targeting extracellular matrix are often enzymes (hyaluronidase, collagenase and elastase etc.), which can digest the constituents. Toxins targeting the plasma membrane affect its permeability and interfere with its transmembrane signaling system. Toxins targeting components in the cytoplasm also interfere with signaling systems or cytoskeleton. Toxins that cause dysfunctions in the immune system are so-called superantigens.

The different classification systems are unfortunately a source of confusion, because one toxin can have different names depending on the applied classification system. The botulinum toxin e.g. is both a neurotoxin and an AB toxin. Abbreviations are often used and the botulinum toxin for instance can be designated BoNT.


A toxoid is a protein toxin, which has been denatured (by e.g. heat or chemical treatment) and has, therefore, lost its toxicity. A toxoid retains, however, its antigenic properties and can, therefore, be used in vaccines.


LPS does not exist in gram positive bacteria, but except LPS, there are some other endotoxins and one example is the Cry protein in Bacillus thuringiensis, which is a so-called δ-endotoxin. Endotoxins are in general much less toxic than exotoxins.

Secretion systems

Bacteria have complicated secretion systems to be able to export proteins to the surrounding or to the cytoplasm of cells of the host animal. Proteins, which are exported are often toxins. Information about secretion systems have also been included in the term list. See Secretion system, bacterial.

Updated: 2019-01-25.

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