Bacillus thuringiensis produces crystals of toxin that are
lethal to larval stages (caterpillars) of the insects which belong
to orders Coleoptera, Lepidoptera and Diptera, but are considered
harmless to most other organisms, including humans. The spores of
the bacterium and the toxic crystals that are produced in the
bacterial plasmids are used in crop protection.
Cry toxins have specific activities against insect species of the
orders Lepidoptera (moths and butterflies), Diptera (flies and
mosquitoes), Coleoptera (beetles), Hymenoptera (wasps, bees, ants
and sawflies) and nematodes. Thus, B. thuringiensis serves
as an important reservoir of Cry toxins for production of
biological insecticides and insect-resistant genetically modified
crops.
When insects ingest toxin crystals, the alkaline pH of their
digestive tract denatures the insoluble crystals, making them
soluble and thus amenable to being cut with proteases found in the
insect gut, which liberate the cry toxin from the crystal. The Cry
toxin is then inserted into the insect gut cell membrane, forming a
pore. The pore results in cell lysis and eventual death of the
insect.
Bacillus thurigiensis-based insecticides are often applied
as liquid sprays on crop plants, where the insecticide must be
ingested to be effective. The gene encoding the Bt toxin is
commonly used in modern biotechnology to introduce the resistance
to crop plants, such as maize and cotton.
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