B. 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
B. 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.