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Botulinum Toxin (BTX) is produced by three species of obligate anaerobe bacterium, primarily ''Clostridium botulism'', but ''Clostridium baratii'' and ''Clostridium butyricum'' also produce the protein <ref>Hall JD, McCroskey LM, Pincomb BJ, Hatheway CL. Isolation of an organism resembling Clostridium baratii which produces type F botulinal toxin from an infant with botulism. J Clin Microbiol. 1985;21:654-655. 36. </ref> <ref>Aureli P, Fenicia L, Pasolini B, Gianfranceschi M, McCroskey LM, Hatheway CL. Two cases of type E infant botulism caused by neurotoxigenic Clostridium butyricum in Italy. J Infect Dis. 1986;154: 207-211. 37. </ref> <ref>Arnon SS. Botulism as an intestinal toxemia. In: Blaser MJ, Smith PD, Ravdin JI, Greenberg HB, Guerrant RL, eds. Infections of the Gastrointestinal Tract. New York, NY: Raven Press; 1995:257-271.</ref>. ''Clostridium botulism'' is commonly found in soil, marine sediments, and the gut of grazing animals <ref>Ward BQ, Carroll BJ, Garrett ES, GB Reese. Survey of the U.S. Gulf Coast for the presence of Clostridium botulinum. Appl Microbiol. 1967;15:629–636. 26.</ref> <ref> Smith LDS. The occurrence of Clostridium botulinum and Clostridium tetani in the soil of the United States. Health Lab Sci. 1978;15:74–80. 27. </ref> <ref>Sugiyama H. Clostridium botulinum neurotoxin. Microbiol Rev. 1980;44:419–448. 28. Dodds KL. Clostridium botulinum in the environment. In: Hauschild AHW </ref> <ref>Dodds KL, eds. Clostridium botulinum—Ecology and Control in Foods. New York, NY: Marcel Dekker, Inc; 1992: 21–51. 29. </ref> <ref>Popoff MR. Ecology of neurotoxigenic strains of clostridia. In: Montecucco C, ed. Current Topics in Microbiology: Clostridial Neurotoxins. The Molecular Pathogenesis of Tetanus and Botulism. Vol 195. Berlin, Germany: Springer-Verlag; 1995: 1–29. </ref> .BTX is only produced by these bacteria when there are anaerobic conditions that induce spore germination. Once produced, the protein can have toxic effects on animals and humans by causing botulism, a potentially fatal illness. BTX is also the active component of Botox.  

There are seven forms of the protein, named A through G, that are structurally similar but create different immune responses <ref>  Hatheway cL. Clostridium botulinum and other clostridia that produce botulinum neurotoxins. in: Hauschild aHW, Dodds kL, eds. Clostridium botulinum—Ecology and Control in Foods. new york, ny: marcel Dekker, inc; 1992: 3–10 </ref>. All seven structures are produced as a single chain protein in the bacterium, but becomes active when a protease cuts the protein into a heavy and light chain connected by a single disulfide bond. The heavy chain is approximately 100 kDa and the light chain is 50 kDa (for reviews about structure see references <ref>Sakaguchi G. 1983. Clostridium botulinum toxins. Pharmacol. Ther. 19:165–

94. </ref> <ref> Minton NP. 1995. Molecular genetics of

Immunol. 195:161–94 </ref> <ref> Oguma K, Fujinaga Y, Inoue K. 1995.Structure and function of Clostridium botulinum toxins. Microbiol. Immunol. 39:161–68 </ref> <ref name="Lacy"> PMID: 3722863</ref> <ref>  Popoff MR, Marvaud J-C. 1999. Structural and genomic features of clostridial neurotoxins. See Ref. 132, pp. 174–

201 </ref>).

BTX also has two auxiliary proteins that compromise a multimeric complex: hemagglutinins (HA) and nonhemagglutinin (NTNH). HA and NTNH do not directly play a role in the toxic effect of BTX, but have an indirect role during ingestion of the protein by making the BTX more resistant to low pH environments and proteolytic enzymes found in the gut.  

The active form of BTX has three functional domains responsible for binding, translocation, and catalysis <ref>Montecucco and Schiavo 1995 C. Montecucco, G. Schiavo Structure and function of tetanus and botulinum neurotoxins Quart. Rev. Biophys., 28 (1995), pp. 423–472. </ref>. The C terminus of the heavy chain is responsible for binding a ganglioside and protein receptors on the nerve terminal while the N terminus half functions by translocating the protein across the membrane by signaling endocytosis <ref>Dolly et al 1984 J.O. Dolly, J. Black, R.S. Williams, J. Melling Acceptors for botulinum neurotoxin reside on motor nerve terminals and mediate its internalization Nature, 307 (1984), pp. 457–460 </ref>. Once in a vesicle in the cytosol of a nerve terminal, the light chain dissociates from the heavy chain and cleaves SNARE proteins involved in synaptic vesicle fusion. The cleavage of these SNARE proteins, VAMP, SNAP-25, and syntax, depends on the serotype of BTX where each serotype cuts at a different place. The cut SNAREs inhibit the binding of vesicles containing acetylcholine which effectively stops the signaling from nerves to muscle cells, resulting in paralysis <ref name="Lacy" />

Of the seven serotypes of BTX, the forms that most often cause botulism in humans are A, B, and E <ref> arnon SS, Schechter r, inglesby tV, et al. botulinum toxin as a biological weapon: medical and public health management. JAMA. 2001;285:1059–1070.</ref>. Humans can be exposed to the neurotoxin through inhalation, ingestion, or surface wounds. Within 12 to 72 hours of exposure to BTX, the inhibition of acetylcholine signaling at nerve synapses creates symptoms of vomiting, nausea, diarrhea, blurred vision, ptosis, dysarthria and dysphagia, and finally weakness of muscles beginning with the head and moving down to the lower extremities <ref> Dembek, Z. F.; Smith, L. A.; Rusnak, J. Botulinum Toxin. In Medical Aspects of Biological Warfare; 2007.</ref>. If not treated, BTX can cause death due to respiratory failure or due to complications when on prolonged medical ventilation.

 

The lethal doses for a human weighing 70 kg is 0.09-0.15 μg when administered intravenously or intramuscularly, 0.70 - 0.90 μg through inhalation, and 70 μg orally <ref>  Franz DR, Pitt LM, Clayton MA, Hanes MA, Rose KJ. Efficacy of prophylactic and therapeutic administration of antitoxin for inhalation botulism. In: DasGupta BR, ed. Botulinum and Tetanus Neurotoxins: Neurotransmission and Biomedical Aspects. New York, NY: Plenum Press; 1993:473-476. </ref> <ref>  Herrero BA, Ecklung AE, Streett CS, Ford DF, King JK. Experimental botulism in monkeys: a clinical pathological study. Exp Mol Pathol. 1967;6:84-95.</ref>.  

The treatment for human exposure to BTX is an antitoxin developed from horse plasma. Horses are exposed naturally to "Clostridium botulism" in their gut and therefore produce antibodies against "Clostridium botulism" in case the bacteria population grows to an extreme. Antitoxins have been developed by isolating this antibody from horses, and in some cases, removing the Fc from the immunoglobulin and leaving the F(ab')2 fragment. By removing the Fc fragment, the incidence of hypersensitivity and anaphylaxis. There are divalent, trivalent, and heptavalent antitoxins available that treat botulinum serotypes A and B (divalent), serotypes A, B, and C (trivalent), and all seven serotypes (heptavalent). An antitoxin, botulism immune globulin (BabyBIG) has also been developed that can be used in infantile cases of botulism. BabyBIG is produced by using the human antibodies produced when a person is injected with pentavalent botulinum toxoid vaccine. By using human antibodies, there is less of a risk of anaphylactic shock and hypersensitivity to equine antigens <ref> Dembek, Z. F.; Smith, L. A.; Rusnak, J. Botulinum Toxin. In Medical Aspects of Biological Warfare; 2007.</ref>.

The antitoxins cannot stop BTX from inhibiting acetylcholine signaling once it has started in parts of the body, but it can stop the progression of paralysis. Thus, treatment can decrease fatalities to 5-10% if administered within 24 hours of exposure <ref> Dembek, Z. F.; Smith, L. A.; Rusnak, J. Botulinum Toxin. In Medical Aspects of Biological Warfare; 2007.</ref>.

Due to the powerful toxicity of BTX, the protein could be used as a biological weapon. The countries that have developed BTX to be used in warfare include Japan, Germany, United States, Russia, and Iraq <ref> Dembek, Z. F.; Smith, L. A.; Rusnak, J. Botulinum Toxin. In Medical Aspects of Biological Warfare; 2007. </ref>.

BTX has also been used medically and cosmetically as Botox. Various disorders characterized by involuntary muscle contraction can be treated with a small injection of BTX to weaken the spastic muscles for three to four months <ref>Nayyar, P.; Kumar, P.; Nayyar, P. V.; Singh, A. BOTOX: Broadening the Horizon of Dentistry. J. Clin. Diagn. Res. JCDR 2014, 8 (12), ZE25–ZE29. </ref>. Some of these diseases include

cervical dystonia, blepharospasm, strabismus, chronic migraines, primary axillary hyperhidrosis, oesophageal achalasia, vaginismus, and detrusor overactivity among others.  

Botulinum toxin is also used in small quantities to prevent the formation of wrinkles. Small muscles in the face create wrinkles after years of repeated contraction <ref>Botulinum Toxin Injection for Facial Wrinkles - American Family Physician http://www.aafp.org/afp/2014/0801/p168.html#sec-1 (accessed Apr 28, 2016).</ref>. By injecting botulinum toxin into specific muscles, contraction is inhibited and wrinkles are prevented. Continual injections are needed every few weeks because the SNAP-25 proteins regenerate allowing signaling for muscle contraction to begin again.