An exotoxin is a soluble protein excreted by a microorganism, including bacteria, fungi, algae, and protozoa. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl A microorganism (also spelled micro organism or micro-organism and also called a microbe) is an Organism that is Microscopic (usually The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have A fungus (ˈfʌŋgəs is a eukaryotic Organism that is a member of the kingdom Fungi (ˈfʌndʒaɪ Algae ( sing. alga are a large and diverse group of simple typically Autotrophic organisms ranging from Unicellular to Multicellular forms Protozoa (in Greek πρῶτον proton "first" and ζῷα zoia "animals" are unicellular Eukaryotes (singular An exotoxin can cause damage to the host by destroying cells or disrupting normal cellular metabolism. Metabolism is the set of Chemical reactions that occur in living Organisms in order to maintain Life. Both gram negative and gram positive bacteria produce exotoxins. Gram-negative bacteria are those Bacteria that do not retain Crystal violet dye in the Gram staining protocol Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. 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. Endotoxins (not to be confused with Enterotoxin) are potentially toxic, natural compounds found inside Pathogens such as Bacteria.

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. Botulinum toxin is a Neurotoxin Protein produced by the Bacterium Clostridium botulinum. Clostridium botulinum is a Gram-positive, rod shaped bacterium that produces the Neurotoxin Botulin, which causes the flaccid Corynebacterium diphtheriae is a pathogenic Bacterium that causes Diphtheria. Diphtheria ( Greek διφθερα ( diphthera)—“pair of leather scrolls" is an upper respiratory tract illness characterized by sore

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). Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor

Contents 1 Types 1.1 Type I toxins: toxins that act from the cell surface 1.1.1 Superantigens 1.1.2 Heat-stable enterotoxins 1.2 Type II toxins: membrane damaging toxins 1.2.1 Channel-forming toxins 1.2.2 Toxins that enzymatically damage the membrane 1.3 Type III toxins: intracellular toxins 1.3.1 AB toxins 1.3.2 Injected toxins 1.4 Toxins that damage the extracellular matrix 2 References 3 See also 4 External links

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. Superantigens (SAgs are secreted proteins ( Exotoxins) that exhibit highly potent lymphocyte-transforming ( Mitogenic activity directed towards T Lymphocytes The best characterized superantigens are those produced by the strains of Staphylococcus aureus and Streptococcus pyogenes that cause toxic shock syndrome. Staphylococcus aureus (ˌstæfɨləˈkɒkəs ˈɔriəs literally "Golden Cluster Seed" and also known as golden staph) is the most common cause of Streptococcus pyogenes is a spherical Gram-positive Bacteria that grows in long chains and is the cause of Group A streptococcal infections Toxic shock syndrome ( TSS) is a rare but potentially fatal disease caused by a bacterial toxin. 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. MHC ( Major histocompatibility complex) Class II molecules are found only on a few specialized cell types including Macrophages Dendritic cells and See also Antigen presentation An antigen-presenting cell ( APC) or accessory cell is a cell that displays foreign Antigen complexed The T cell receptor or TCR is a molecule found on the surface of T lymphocytes (or T cells that is in general responsible for recognizing Antigens bound T cells belong to a group of White blood cells known as Lymphocytes, and play a central role in Cell-mediated immunity. Consequently, up to 20% of all T cells are activated, leading to massive secretion of proinflammatory cytokines, which produce the symptoms of toxic shock. Cytokines are a category of signalling Proteins and Glycoproteins that like Hormones and Neurotransmitters, are used extensively in cellular

Heat-stable enterotoxins

Some strains of E. coli produce heat-stable enterotoxins (ST), which are small peptides that are able to withstand heat treatment at 100^oC. Heat-stable enterotoxins are secretory peptides produced by some bacterial strains 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. Cyclic guanosine monophosphate ( cGMP) is a Cyclic nucleotide derived from Guanosine triphosphate (GTP 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. Hemolysins are Exotoxin Protein produced by Bacteria which causes lysis of Red blood cells 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 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. Pore forming toxins (PFTs are Protein Toxins, typically (but not exclusively produced by Bacteria, such as ''C

• 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. Staphylococcus aureus (ˌstæfɨləˈkɒkəs ˈɔriəs literally "Golden Cluster Seed" and also known as golden staph) is the most common cause of Cholesterol is a Lipid found in the Cell membranes and transported in the Blood plasma of all Animals It is an essential component of mammalian 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. Secretion is the process of segregating elaborating and releasing chemicals from a cell, or a secreted Chemical substance or amount of substance ^[1] The exception is pneumolysin, which is released from the cytoplasm of Streptococcus pneumoniae when the bacteria lyse. Streptococcus pneumoniae, or pneumococcus, is a Gram-positive, alpha-hemolytic diplococcus aerotolerant anaerobe and a member of the Genus Pneumolysin, Clostridium perfringens perfringolysin, and Listeria monocytogenes listeriolysin O cause specific modifications of histones in the host cell nucleus, resulting in down-regulation of several genes encoding proteins involved in the inflammatory resopnse. Clostridium perfringens (formerly known as Clostridium welchii) is a Gram-positive, rod-shaped anaerobic, spore-forming Bacterium Listeria monocytogenes is a Gram-positive Bacterium, in the division Firmicutes, named for Joseph Lister. Listeriolysin O (LLO is a Hemolysin produced by the Bacterium Listeria monocytogenes, the pathogen responsible for causing Listeriosis In Biology, histones are the chief Protein components of Chromatin. ^[2] Histone modification does not involve the pore-forming activity of the CDCs.

• RTX toxins

RTX (repeats in toxin) 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. Clostridium perfringens (formerly known as Clostridium welchii) is a Gram-positive, rod-shaped anaerobic, spore-forming Bacterium Gas gangrene is a Bacterial Infection that produces Gas within tissues in Gangrene. α toxin has phospholipase activity. A phospholipase is an enzyme that converts Phospholipids into Fatty acids and other Lipophilic substances

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. Enzymes are Biomolecules that catalyze ( ie increase the rates of Chemical reactions Almost all enzymes are Proteins The structure of these toxins allows for the development specific vaccines and treatments. A vaccine is a biological preparation which is used to establish or improve immunity to a particular disease Certain compounds can be attached to the B unit, which is not generally harmful, which the body learns to recognize, and which elicits an immune response. Immunity is a material term that describes a state of having sufficient biological defenses to avoid Infection, Disease, or other unwanted biological invasion 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. Cholera toxin (sometimes abbreviated to CTX, Ctx, or CT) is a Protein complex secreted by the Bacterium Vibrio cholerae Pertussis toxin (PT is a protein-based AB5-type Exotoxin produced by the Bacterium Bordetella pertussis. Shiga toxins are a family of related toxins with two major groups Stx1 and Stx2 whose genes are considered to be part of the Genome of lambdoid Prophages An enterotoxin (not to be confused with Endotoxin) is a Protein Toxin released by a Microorganism in the Intestine.

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. Secretion is the process of segregating elaborating and releasing chemicals from a cell, or a secreted Chemical substance or amount of substance Yersinia is a Genus of bacteria in the family Enterobacteriaceae.

Toxins that damage the extracellular matrix

These "toxins" allow the further spread of bacteria and consequently deeper tissue infections. Examples are hyaluronidase and collagenase. The hyaluronidases ( are a family of Enzymes that degrade Hyaluronic acid. Collagenases are Enzymes that break the Peptide bonds in Collagen. 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. ^[3]

References

1. ^ Tweten RK (2005). "Cholesterol-dependent cytolysins, a family of versatile pore-forming toxins". Infect. Immun. 73 (10): 6199-209. doi:10.1128/IAI.73.10.6199-6209.2005. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 16177291.  
2. ^ Hamon MA, Batsché E, Régnault B, Tham TN, Seveau S, Muchardt C, Cossart P (2007). "Histone modifications induced by a family of bacterial toxins". Proc. Natl. Acad. Sci. U. S. A. 104 (33): 13467-72. doi:10.1073/pnas.0702729104. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 17675409.  
3. ^ Bauman, Robert (2004)Microbiology ISBN 0-8053-7590-2

See also

• Infectious disease
• Endotoxin
• Mycotoxin
• Toxin

External links

• MeSH Exotoxins

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