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 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.
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.