Exotoxin 503254 220724055 2008-06-21T07:26:56Z DOI bot 6652755 Citation maintenance. Initiated by [[User:Fconaway|Fconaway]]. You can [[WP:DOI|use this bot]] yourself! Please [[User:DOI_bot/bugs|report any bugs]]. An '''exotoxin''' is a soluble [[protein]] excreted by a [[microorganism]], including [[bacterium|bacteria]], [[fungi]], [[algae]], and [[protozoa]]. An exotoxin can cause damage to the host by destroying cells or disrupting normal [[cellular metabolism]]. Both [[gram negative]] and [[gram positive]] bacteria produce exotoxins. They are highly potent and can cause major damage to the host. Exotoxins may be secreted, or, similar to [[endotoxins]], may be released during lysis of the cell. Most exotoxins can be destroyed by heating. They may exert their effect locally or produce systemic effects. (Nester, 2007). Well known exotoxins include the [[botulinum toxin]] produced by ''[[Clostridium botulinum]]'', the ''[[Corynebacterium diphtheriae]]'' exotoxin which is produced during life threatening symptoms of [[diphtheria]]. Exotoxins are susceptible to [[antibodies]] produced by the [[immune system]], but many exotoxins are so toxic that they may be fatal to the host before the immune system has a chance to mount defenses against it (Nester, 2007). ==Types== Although many exotoxins can be categorized by their mode of action on target cells, rigid classification of some toxins is not possible or appropriate. ===Type I toxins: toxins that act from the cell surface=== Type I toxins bind to a receptor on the cell surface and stimulate intracellular signaling pathways. Two examples are described below. ====='''Superantigens'''===== [[Superantigens]] are produced by several bacteria. The best characterized superantigens are those produced by the strains of ''[[Staphylococcus aureus]]'' and ''[[Streptococcus pyogenes]]'' that cause [[toxic shock syndrome]]. Superantigens bridge the [[MHC class II]] protein on [[antigen presenting cells]] with the [[T cell receptor]] on the surface of [[T cells]] with a particular Vβ chain. Consequently, up to 20% of all T cells are activated, leading to massive secretion of proinflammatory [[cytokines]], which produce the symptoms of toxic shock. ====='''Heat-stable enterotoxins'''===== Some strains of ''[[E. coli]]'' produce [[heat-stable enterotoxin]]s (ST), which are small peptides that are able to withstand heat treatment at 100<sup>o</sup>C. Different STs recognize distinct receptors on the cell surface and thereby affect different intracellular signaling pathways. For example, STa enterotoxins bind and activate membrane-bound guanylate cyclase, which leads to the intracellular accumulation of [[cyclic GMP]] and downstream effects on several signaling pathways. These events lead to the loss of electrolytes and water from intestinal cells. ===Type II toxins: membrane damaging toxins=== Membrane damaging toxins exhibit [[hemolysin]] or cytolysin activity ''in vitro''. However, induction of cell lysis may not be the primary function of the toxins during infection. At low concentrations of toxin, more subtle effects such as modulation of host cell signal transduction may be observed in the absence of cell lysis. Membrane-damaging toxins can be divided into two categories, the channel-forming toxins and toxins that function as enzymes that act on the membrane. ====='''Channel-forming toxins'''===== Most [[Pore forming toxins|channel-forming toxins]], which form pores in the target cell membrane, can be classified into two families, the cholesterol-dependent toxins and the RTX toxins. *'''Cholesterol-dependent cytolysins''' Formation of pores by cholesterol-dependent cytolysins (CDC) such as the α toxin of ''[[Staphylococcus aureus]]'' requires the presence of [[cholesterol]] in the target cell. The size of the pores formed by members of this family is extremely large: 25-30 nm in diameter. A conserved 11 amino acid sequence is found at the C-terminus of all family members. Moreover, all CDCs are secreted by the type II [[secretion]] system.<ref name="pmid16177291">{{cite journal |author=Tweten RK |title=Cholesterol-dependent cytolysins, a family of versatile pore-forming toxins |journal=Infect. Immun. |volume=73 |issue=10 |pages=6199–209 |year=2005 |pmid=16177291 |doi=10.1128/IAI.73.10.6199-6209.2005}}</ref> The exception is [[pneumolysin]], which is released from the cytoplasm of ''[[Streptococcus pneumoniae]]'' when the bacteria lyse. Pneumolysin, ''[[Clostridium perfringens]]'' [[perfringolysin]], and ''[[Listeria monocytogenes]]'' [[listeriolysin O]] cause specific modifications of [[histone|histones]] in the host cell [[nucleus]], resulting in down-regulation of several genes encoding proteins involved in the inflammatory resopnse.<ref name="pmid17675409">{{cite journal |author=Hamon MA, Batsché E, Régnault B, Tham TN, Seveau S, Muchardt C, Cossart P |title=Histone modifications induced by a family of bacterial toxins |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=104 |issue=33 |pages=13467–72 |year=2007 |pmid=17675409 |doi=10.1073/pnas.0702729104}}</ref> Histone modification does not involve the pore-forming activity of the CDCs. *'''RTX toxins''' RTX (''r''epeats in ''t''o''x''in) cytolysins can be identified by the presence of a specific tandemly-repeated nine amino acid residue sequence in the protein. The prototype RTX member is the HlyA hemolysin of ''E. coli''. ====='''Toxins that enzymatically damage the membrane'''===== One example is the α toxin of ''[[Clostridium perfringens]]'', which causes [[gas gangrene]]. α toxin has [[phospholipase]] activity. ===Type III toxins: intracellular toxins=== Intracellular toxins must be able to gain access to the cytoplasm of the target cell to exert their effects. ====='''AB toxins'''===== One group of intracellular toxins is the AB toxins. The 'B'-subunit attaches to target regions on cell membranes, the 'A'-subunit enters through the membrane and possesses [[enzymatic]] function that affects internal cellular bio-mechanisms. The structure of these toxins allows for the development specific [[vaccine]]s and treatments. Certain compounds can be attached to the B unit, which is not generally harmful, which the body learns to recognize, and which elicits an [[immunity (medical)|immune response]]. This allows the body to detect the harmful toxin if it is encountered later, and to eliminate it before it can cause harm to the host. Toxins of this type include [[cholera toxin]], [[pertussis toxin]], [[Shiga toxin]] and heat-labile [[enterotoxin]] from ''[[E. coli]]''. ====='''Injected toxins'''===== Some bacteria deliver toxins directly from their cytoplasm to the cytoplasm of the target cell through a needle-like structure. The effector proteins injected by the type III [[secretion]] apparatus of ''[[Yersinia]]'' into target cells are one example. ===Toxins that damage the extracellular matrix=== These "toxins" allow the further spread of bacteria and consequently deeper tissue infections. Examples are [[hyaluronidase]] and [[collagenase]]. These molecules, however, are enzymes that are secreted by a variety of organisms and are not usually considered toxins. They are often referred to as virulence factors, since they allow the organisms to move deeper into the hosts tissues.<ref> Bauman, Robert (2004)Microbiology ISBN 0-8053-7590-2 </ref> ==References== <references/> ==See also== * [[Infectious disease]] * [[Endotoxin]] * [[Mycotoxin]] * [[Toxin]] ==External links== * {{MeshName|Exotoxins}} [[Category:Toxins]] [[Category:Bacteriology]] [[Category:Microbiology]] {{micro-stub}} [[de:Ektotoxin]] [[hu:Exotoxin]] [[it:Esotossina]] [[ja:外毒素]] [[pl:Egzotoksyna]] [[pt:Exotoxina]] [[sr:Егзотоксин]]