- Some strains of Bacillus thuringiensis produce protein crystals during a particular phase of their growth.
- The crystals contain a toxic insecticidal protein that kill certain insects such as lepidopterans (tobacco budworm, armyworm), coleopterans (beetles) and dipterans (flies, mosquitoes).
- The toxin does not kill the Bacillus because the Bt toxin protein exists as inactive protoxins but once the insect ingests the inactive toxin, it is converted into an active form
of toxin due to the alkaline pH of the gut which solubilize the crystals.
- The activated toxin binds to the surface of midgut epithelial cells and create pores that cause cell swelling and lysis and cause death of the insect.
- The toxin is coded by a gene named cry.
- There are a number of genes acting against the insects , for example, the proteins encoded by the genes cryIAc and cryIIAb control the cotton bollworms, that of cryIAb controls corn borer.
- Specific Bt toxin genes were isolated from Bacillus thuringiensis based on the crop and the targeted pest which is then incorporated into the several crop plants.
Fig. Bt cotton plants
Fig. Bacillus thuringiensis