Why do Bt toxin protein crystals present in the bacterium Bacillus thuringiensis not kill the bacteria?

Toxins occur as inactive protoxins in bacteria.

The correct answer is Option (3)- Toxins occur as inactive protoxins in bacteria.

Bacillus thuringiensis (Bt) is a bacterium that produces proteins called Bt toxins, which are toxic to certain insects. These toxins have been utilized in biotechnology to create pest-resistant plants, reducing the need for chemical insecticides. One such example is Bt cotton, where the Bt toxin gene is cloned from the bacterium and expressed in the plant.

Some strains of Bacillus thuringiensis produce proteins that kill certain insects such as lepidopterans (tobacco budworm, armyworm), coleopterans (beetles) and dipterans (flies, mosquitoes).

The Bt toxins are produced in the form of protein crystals during a specific phase of B. thuringiensis growth. These crystals contain an inactive form of the toxin, known as protoxins. Interestingly, the toxin does not harm the bacterium itself because the toxin is in its inactive form.

When an insect ingests the inactive toxin, it reaches the alkaline environment of the insect's gut. The alkaline pH of the gut solubilizes the crystals, converting the protoxins into their active toxin form. The activated toxin then binds to the surface of the midgut epithelial cells of the insect. Once bound, the toxin creates pores in the cell membrane, causing the cells to swell and eventually lyse (burst). This disruption in the midgut epithelium leads to the insect's death.

By engineering plants to express the Bt toxin, farmers can now grow crops that are naturally resistant to certain insects. When insects attempt to feed on these Bt-expressing plants, they ingest the toxin and suffer the consequences, thus reducing the damage caused by pests and decreasing the need for chemical insecticides. This approach has been widely used in agriculture to protect crops and promote more sustainable farming practices.