The reason Bt toxin is not toxic to the bacteria producing it is because the Bt toxin protein exists as inactive protoxins inside the Bacillus bacteria.

Bacillus thuringiensis (Bt) is a bacterium that naturally produces a protein called Bt toxin, which is harmful to certain insect pests. However, within the Bacillus bacterium, the Bt toxin exists in an inactive form known as protoxins. These protoxins are not toxic to the bacteria itself.

The Bt toxin remains in an inactive state inside the bacterial cells until it is ingested by specific susceptible insect larvae or pests. When the protoxin is consumed by the target insect, it undergoes activation by the alkaline pH environment and enzymes present in the insect's gut. The activated form of the toxin binds to receptors in the insect's digestive system, leading to the disruption of the insect's gut lining, ultimately causing death.

Therefore, the correct reason why the Bt toxin is not toxic to the bacteria producing it is that the Bt toxin protein exists as inactive protoxins inside the Bacillus bacterium.