Why does Bt toxin not till the bacterium that produces it, but kills the insect that ingests it?
Bt toxin is produced by a soil bacterium called Bacillus thuringiensis. This toxin does not kill the bacterium which produces it, because when it is present inside it in an inactive and crystalline form. It becomes active and toxic only when it is consumed by insects such as lepidopterans etc., due to alkaline pH of the gut.