Clostridium botulinum and the Most Poisonous Poison

Abstract

Botulinum neurotoxin (BoNT) is the most poisonous toxin known to humankind and is the cause of the neuroparalytic disease botulism in humans and animals. Due to the extraordinary toxicity of BoNT, control of botulism is a perennial concern of the food industry, medicine, and regulatory agencies. BoNT also poses a serious concern as a bioterrorism agent with the potential of causing mass casualties, and BoNT is designated as a Category A Select Agent, the most severe group comprising six agents, which also includes anthrax toxins and hemorrhagic viruses. There are seven serotypes of BoNT (designated A–G), which are defined by their neutralization against death in mice by homologous antibodies. BoNTs are produced by neurotoxigenic clostridia, particularly the diverse bacterial group Clostridium botulinum, and by sporadic strains of Clostridium argentinense, Clostridium butyricum, Clostridium baratii, and Clostridium sporogenes. These clostridia produce heat and chemical resistant endospores, which are widespread in soils, freshwater and marine sediments, and the gastrointestinal tract of certain animals (but not humans). Surveys have revealed that C. botulinum spores are present in the USA in about 35% of soil samples examined. BoNTs are ~150 kDa proteins comprised of a Heavy Chain (HC) and a Light Chain (LC). During the intoxication process, the HC binds selectively and with high affinity to peripheral nerve terminals, and the LC enters into the cytosol and inhibits the presynaptic release of acetylcholine, causing postsynaptic denervation of muscle and a characteristic flaccid paralysis that can last for several weeks to months in humans. BoNTs also cause disturbances in the sensory and autonomic nervous systems. The recognition of diversity of BoNTs has expanded in recent years mainly through DNA sequencing efforts, and within the seven serotypes (A-G) there are more than 40 sequence variants (subtypes) of BoNTs currently recognized (Hill et al. 2013. BoNTs are produced naturally in protein complexes, in which nontoxic associated proteins protect the heat-labile BoNT component in the digestive tract of humans and animals and likely in the environment. Six categories of botulism are recognized: (1) foodborne botulism whereby preformed toxin (and sometimes cells and spores) are ingested in foods, (2) infant botulism in which C. botulinum colonizes the infant gut and produces BoNTs, (3) adult intestinal botulism analogous to infant botulism, (4) wound botulism due to colonization and BoNT formation in surface wounds, (5) inhalational botulism which is extremely rare naturally but a concern of bioterrorism, and (6) iatrogenic botulism caused by poisoning from inappropriate injection of BoNT for pharmaceutical and aesthetic purposes. In all six categories, the symptoms and pathology are very similar. BoNT is absorbed into the lymphatic system and then into the blood stream, and is then transported to peripheral nerve terminals, primarily cranial and somatic nerves. BoNT is internalized into the nerve terminals at the neuromuscular junction and prevents the release of acetylcholine to muscles innervating the eyes, face, and mouth, and then the toxicity descends bilaterally to motor nerves of the torso causing a generalized paralysis. There is no antidote for botulism once it enters nerves, and currently the only means for treatment of the disease is by thorough nursing care, and in severe cases mechanical ventilation and parenteral feeding. Serum therapy by administration of human (BabyBIG®) in infants or heptavalent equine antibodies (currently HBAT) in adults into the circulation of poisoned toddlers and adults, can decrease the severity of the paralysis and duration of the illness while BoNT remains in the blood stream. This chapter focuses on aspects of neurotoxigenic clostridia and botulinum neurotoxins that have not been extensively reviewed recently, including the importance of BoNTs as a public health risk, properties of BoNT-producing strains and BoNTs, laboratory criteria needed for a definitive diagnosis of botulism, the pathology, recovery and treatment of botulism, and lastly a brief section on strategies to prevent foodborne and infant botulism.