Exotoxin

An exotoxin is a toxin excreted by a microorganism, like bacteria, fungi, algae, and protozoa. An exotoxin can cause damage to the host by destroying cells or disrupting normal cellular metabolism. 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. Well-known exotoxins include the botulinum toxin produced by Clostridium botulinum and 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.

Types
Many exotoxins have been categorized

This classification, while fairly exhaustive, is not the only system used. Other systems for classifying or identifying toxins include:
 * By organism generating the toxin
 * By organism susceptible to the toxin
 * By tissue target type susceptible to the toxin (neurotoxins affect the nervous system, cardiotoxins affect the heart, etc.)
 * By structure (for example, AB5 toxin)
 * By the ability of the toxin to endure in hostile environments, such as heat, dryness, radiation, or salinity. In this context, "labile" implies susceptibility, and "stable" implies a lack of susceptibility.
 * By a letter, such as "A", "B", or "C", to communicate the order in which they were identified.

The same exotoxin may have different names, depending of the field of research.

Type I: cell surface-active
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. As a consequence, 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 enterotoxins (ST), which are small peptides that are able to withstand heat treatment at 100oC. 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: membrane damaging
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 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 perfringolysin O of Clostridium perfringens 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. 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 histones in the host cell nucleus, resulting in down-regulation of several genes encoding proteins involved in the inflammatory response. 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. RTX is also found in Legionella pneumophila.

Enzymatically Active Toxins
One example is the α toxin of C. perfringens, which causes gas gangrene; α toxin has phospholipase activity.

Type III: intracellular
Type III exotoxins can be classified by their mode of entry into the cell, or by their mechanism once inside.

By mode of entry
Intracellular toxins must be able to gain access to the cytoplasm of the target cell to exert their effects.


 * 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.


 * Another group of intracellular toxins is the AB toxins. The 'B'-subunit (binding) attaches to target regions on cell membranes, the 'A'-subunit (active) enters through the membrane and possesses enzymatic function that affects internal cellular bio-mechanisms. A common example of this A-subunit activity is called ADP-ribosylation in which the A-subunit catalyzes the addition of an ADP-ribose group onto specific residues on a protein.  The structure of these toxins allows for the development of specific vaccines and treatments. Certain compounds can be attached to the B unit, which is not, in general, harmful, which the body learns to recognize, and which elicits an 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-liable enterotoxin from E. coli.

By mechanism
Once in the cell, many of the exotoxins act at the eukaryotic ribosomes (especially 60S), as protein synthesis inhibitors. (Ribosome structure is one of the most important differences between eukaryotes and prokaryotes, and, in a sense, these exotoxins are the bacterial equivalent of antibiotics such as clindamycin.)


 * Some exotoxins act directly at the ribosome to inhibit protein synthesis. An example is Shiga toxin.


 * Other toxins act at elongation factor-2. In the case of the diphtheria toxin, EF2 is ADP-ribosylated and becomes unable to participate in protein elongation, and, so, the cell dies. Pseudomonas exotoxin has a similar action.

Other intracellular toxins do not directly inhibit protein synthesis.
 * For example, Cholera toxin ADP-ribosylates, thereby activating tissue adenylate cyclase to increase the concentration of cAMP, which causes the movement of massive amounts of fluid and electrolytes from the lining of the small intestine and results in life-threatening diarrhea.


 * Another example is Pertussis toxin.

Extracellular matrix damage
These "toxins" allow the further spread of bacteria and, as a consequence, 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.