Bacillus thuringiensis (Bt) is a naturally occurring, soil-borne bacteria that has been used since the 1950s for natural insect control. It consists of a spore, which gives it persistence, and a protein crystal within the spore, which is toxic. That toxic protein differs, depending on the subspecies of Bt-producing it, yielding a variance of Bt toxic to different insect species (or none at all). When the bacteria is consumed by certain insects, the toxic crystal is released in the insects highly alkaline gut, blocking the system which protects the pest’s stomach from its own digestive juices. The stomach is penetrated, and the insect dies by poisoning from the stomach contents and the spores themselves. This same mechanism is what makes Bt harmless to birds, fish and mammals whose acidic gut conditions negate the bacteria’s effect. Specific Bt toxin genes were isolated from Bacillus thuringiensis and incorporated into the several crop plants such as cotton  The choice of genes depends upon the crop and the targeted pest, as most Bt toxins are insect-group specific.