Explain why in case of Bt. insecticide protein produced by the bacterium kill the insect pest but not the cell of bacterium?

Explanation

The Bt-insecticide protein (cry protein) is produced as a protoxin (inactive) in the bacteria Bacillus thuringiensis by cry genes. So bacteria itself is not harmed by the protein. But once the insect ingests the inactive toxin, it is converted into active toxin due to alkaline pH of the gut, which solubilises the crystals. The activated toxin binds to surface of midgut epithetial cells and create pores which cause cell swelling and lysis and finally cause death of the insect .
So bacteria producing this toxin itself remains unharmed.