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The protein toxins resemble enzymes in a number of ways. Like enzymes, bacterial exotoxins are denatured by heat, acid and proteolytic enzymes;  they have a high biological activity (most act catalytically); and they exhibit specificity of action.
As enzymes attack specific substrates, so bacterial protein toxins are highly specific in the substrate utilized and in their mode of action. The substrate (in the host) may be a component of tissue cells, organs, or body fluid. Usually the site of damage caused by the toxin indicates the location of the substrate for that toxin. Terms such as enterotoxin, neurotoxin, leukocidin or hemolysin are sometimes used to indicate the target site of some well-defined protein toxins.
Certain protein toxins have very specific cytotoxic activity (i.e., they attack specific types of cells). For example, tetanus or botulinum toxins attack only neurons. But some toxins (as produced by staphylococci, streptococci, clostridia, etc.) have fairly broad cytotoxic activity and cause nonspecific death of all sorts of cells and tissues, eventually resulting in necrosis. Toxins that are phospholipases act in this way. They cleave phospholipids which are regular components of host cell membranes, resulting in the death of the cell by leakage of cellular contents. This is also true of pore-forming hemolysins and leukocidins.

Among the 325 protein toxins produced by bacteria so far identified, at least 115 (35%) belong to the group of the so-called membrane-damaging toxins (MDTs). The most characteristic feature of these effectors is to damage or disrupt the cytoplasmic phospholipid bilayer membrane (7–9nm) of appropriate human and (or) animal cells. The impairment of membrane integrity causes osmotic imbalance, reflected by cell swelling due to water influx and dissipation of electrochemical gradients, which may lead to cell lysis and death

10. Ricin
Readily available from castor beans, ricin is an extremely potent toxin. Ricin prevents the synthesis of proteins by disrupting the ribosome, effectively inhibiting the activities of the human body on a cellular level.
Chewing a handful of castor beans can lead to death, and one of the scariest terrorist threats is inhalation of ricin in a purified powder form. If you inhale powdered ricin, respiratory and digestive problems occur within six to eight hours, and death follows in 36 to 72 hours.

9. Cholera toxin
Cholera toxin is interesting from a structural standpoint. The active form of the protein is created from two different subunits that must be assembled correctly in order to create the working toxin. The two subunits have different functions, and when working together, they cause a flood of negatively charged chloride ions, a flood quickly followed by an increase in the presence of positively charged sodium and potassium ions as well as water in the intestine.
Intestinal chaos quickly ensues, with infected individuals becoming dehydrated as the body sheds several liters of fluid in the form of watery diarrhea in hopes of restoring order.

8. Major Prion Protein
Remember "Mad Cow Disease," aka bovine spongiform encephalopathy? This is the protein that causes it. Major prion protein (also known as PrP) is already found throughout the human body, particularly within the nervous system. A slight change in the three-dimensional structure of the protein leads to a variety of debilitating and deadly diseases.

7. Pertussis toxin
Another multi-subunit protein, pertussis toxin shuts down communication between cells inside your body.
Pertussis toxin is released by the bacteria Bordetella pertussis, with the protein interfering with the body's immune system. This combination of a compromised immune system and intercellular communication problems aids Bordetella pertussis in causing cases of whooping cough.

6. Ectatomin
The primary toxic component of venom from Ectatomma tuberculatum ants, ectamonin maneuvers into the plasma membrane of cells. Once settled within the plasma membrane, this relatively small protein (a little less than 8 kDa) creates pores within the membrane. These new pores allow for passage of cations across the membrane and a rapid change in electrical gradient, killing the cell.

5. Conopeptides
A series of short molecules made up from the same amino acid building blocks as proteins, conopeptides originate from poisonous marine cone snails. The snails deliver the peptides through their tooth-like, disposable radula (which you can see on the left).
Conopeptides are a hot topic in pharmacology, with researchers attempting to derive neurological and cardiac drugs using the basic structure of these poisons.

4. Abrin
The lesser known cousin of ricin, abrin is a protein found in seeds of the rosary pea. The CDC labels abrin as a potential agent for chemical terrorism, because it's significantly more potent than ricin and there's no antidote. Vomiting, bloody diarrhea, and hallucinations follow the ingestion of abrin — with death often occurring in three days due to complications from spleen, kidney, and liver failure.

3. Verotoxin
I'm a big fan of this one just because of the name. Verotoxin is often found in cattle feces, making for an easy way to spread the dangerous protein to humans.
This toxin interferes with the creation of proteins within small blood vessels in the kidneys and gastrointestinal tract. Eventually, the damage to the blood vessels will result in decreased organ efficiency or complete organ failure.

2. Tetanospasmin
This neurotoxin eventually causes muscle spasms, the telltale symptom of tetanus. The protein travels through the body with the aim of finding the nervous system. Once in the nervous system, tetanospasmin irreversibly binds to neurons.
While bound, tetanospasmin blocks the release of neurotransmitters by degrading the protein synaptobrevin. This increases muscle sensitivity, leading to the spasms associated with tetanus. Contact with tetanospasmin is extremely deadly, with as little as 200 nanograms capable of causing death in a human. To put this in perspective, an ant weighs about 1500 times the lethal dose of tetanospasmin.

1. Botulinum toxin
First discovered in improperly handled meat products, botulinum toxin is the most toxic substance known to humankind. As little as 100 nanograms of pure botulinum toxin can kill the average human. The onset of symptoms varies from six hours to ten days, with the toxin leading to paralysis of muscles involved in breathing and death.
Botulinum toxin also the protein behind the commercial product Botox, the cosmetic product that paralyzes facial muscles in order to eliminate wrinkles.

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NAME OF TOXIN BACTERIA INVOLVED ACTIVITY
Anthrax toxin (EF) : Bacillus anthracis : sn adenylate cyclase enzyme that increases levels in intracellular cyclic AMP in phagocytes and formation of ion-permeable pores in cell membrane. Leads to edema and decreased phagocytic responses

Adenylate cyclase toxin (pertussis AC) : Bordetella pertussis : Acts locally to increase levels of cyclic AMP in phagocytes and formation of ion-permeable pores in cell membranes

Alpha toxin : Staphylococcus aureus : Protein subunits assemble into an oligomeric structure that forms an ion channel (pore) in the cell plasma membrane
Cholera enterotoxin (Ctx) : Vibrio cholerae : ADP ribosylation of G proteins stimulates adenlyate cyclase and increases cAMP in cells of the GI tract, causing secretion of water and electrolytes leading to diarrhea

E. coli ST toxins Escherichia coli : Binding of the heat-stable enterotoxins (ST) to a guanylate cyclase receptor results in an increase in cyclic GMP (cGMP) that adversely effects electrolyte flux. Promotes secretion of water and electrolytes from intestinal epithelium leading to diarrhea.
Shiga toxin : Shigella dysenteriae, E. coli O157:H7 : Enzymatically cleaves eucaryotic 28S rRNA resulting in inhibition of protein synthesis in susceptible cells. Results in diarrhea, hemorrhagic colitis (HC) and hemolytic uremic syndrome (HUS)

Perfringens enterotoxin : Clostridium perfringens : Stimulates adenylate cyclase leading to increased cAMP in epithelial cells. Result is diarrhea

Botulinum toxin : Clostridium botulinum : Zn++ dependent protease that inhibits neurotransmission at neuromuscular synapses resulting in flaccid paralysis

Tetanus toxin : Clostridium tetani : Zn++ dependent protease that Inhibits neurotransmission at inhibitory synapses resulting in spastic paralysis

Diphtheria toxin (Dtx) : Corynebacterium diphtheriae : ADP ribosylation of elongation factor 2 leads to inhibition of protein synthesis in target cells

Exotoxin A : Pseudomonas aeruginosa : Inhibits protein synthesis; similar to diphtheria toxin

Anthrax toxin (LF) Bacillus anthracis : Lethal Factor (LF) is a Zn++ dependent protease that induces cytokine release and is cytotoxic to cells by an unknown mechanism

Pertussis toxin (Ptx) : Bordetella pertussis : ADP ribosylation of G proteins blocks inhibition of adenylate cyclase in susceptible cells

Exfoliatin toxin* : Staphylococcus aureus : Cleavage within epidermal cells (intraepidermal separation); also acts as a superantigen

Staphylococcus enterotoxins* Staphylococcus aureus : Superantigen causes massive activation of the immune system, including lymphocytes and macrophages; exact role in in emesis not not known

Toxic shock syndrome toxin (TSST-1)* : Staphylococcus aureus :Superantigen acts on the vascular system causing inflammation, fever and shock

Erythrogenic toxin [streptococcal pyrogenic exotoxin (SPE)]* Streptococcus pyogenes Super antigen same as TSST - inflammation, fever and shock; can cause localized erythematous reactions (scarlet fever)



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