Bacillus thuringiensis (Bt) puts out toxic compounds that cause inflammation and tissue damage in the organs of specific insect pests such as caterpillars.
The genes from Bt have been inserted into several genetically modified crops so that the plants produce pesticides themselves. The GM industry argues that this is no different from what organic farmers do when they spray Bt bacteria; however, this is a massive distortion of the facts. Organic farmers spray live bacteria onto the crop, not the Bt toxins. During the night and early morning, pests eat the bacteria, infecting them so that the bacteria release their toxins and eventually kill the pests. It is destroyed by ultraviolet light, so usually, none will survive more than a day or two, and consumers will not be affected by the toxins produced by Bt.
On the other hand, every cell of a Bt GMO plant and its produce contains the Bt toxin, so livestock and people are consuming these pesticide compounds.