Bacillus thuringiensis. This organism, also known as BT, is famous for the production of an insecticidal toxin. The bacterium was initially discovered as a pathogen of various insects and was first used as an insecticidal agent in the early part of this century. This organism, like many other Bacilli, is found in the soil, where it leads a saprophytic existence, but becomes an opportunistic pathogen of insects when ingested. The specific activity of the toxin towards insects and its lack of toxicity to animals has made this organism a useful biocontrol agent. The delta-endotoxin, which is produced during the sporulation part of the life cycle, causes midgut paralysis and disruption of feeding by the infected insect host. The presence of a parasporal crystal, which is outside the exosporium of the endospore, is indicative of production of the toxin, and serves as a marker for this species.
Activation of the toxin typically requires a high pH environment such as the alkaline environments in insect midguts followed by proteolysis. Binding of the toxin to a receptor on intestinal epithelial cells serves to position the toxin to insert itself into the apical membrane where it forms a pore. The flux of water and ions into the cytoplasm results in eventual lysis leading to tissue damage. Various toxin genes specific for a variety of insects have been studied, and many are now being used in genetically modified plants which have been engineered to produce the toxin themselves, eliminating the need to produce sufficient amounts of B. thuringiensis spores.
Bacillus thuringiensis subsp. konkukian serotype H34 strain 97-27. This organism was isolated from a case of severe human tissue necrosis which is unusual since human infections by this organism are rare. This isolate is very closely related to B. anthracis based on phylogenetic analysis.
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