in case of BT cotton how does the toxic insecticide protein produced by the bacterium kill the insect pest but not the cell of bacillus thuringiensis where the toxic protein is generated

Hello Student
Some strains of bacillus thuringiensis produces proteins that kill certain insects as lepidosterans(tobbacobud worm, armyworm). Bt forms protein crystals during a particular phase of their growth. These crystals contains a toxic insectisidal protein but toxin not kill the bacillus because this toxin exists as inactive protoxin but once an insect ingest this inactive toxin it converts into an active form due to the allkaline pH of the gut which solubilise the crystal which cause cell swelling and lysis or eventually cause death of insect.