Question

What causes the inactive form of Bt toxin i.e. Protoxin to get converted into its active form in the body of an insect.?
a. Temperature of the gut
b. Enzymes present in the saliva
c. Alkaline pH of the gut
d. There is no specific reason.

Answer 1

Several types of biological organism or their biochemical products are used in the form of biopesticides. The bacteria and other types of microorganism are widely used for the production of several types of biochemicals or toxins that are fatal to other organisms.

Complete answer:
Bacillus thuringiensis or Bt is a type of Gram-positive bacterium that produces a specific toxin compound which is known as the Bt toxin. The toxin produced by the bacterium is widely used as a biopesticide as it proves to be fatal to the several groups of insects.

The bacterium produces the toxins in the form of crystals of proteinaceous insecticidal endotoxin. This toxin is also called crystal protein or Cry protein. The toxins are produced under the influence of the cry genes which are located on the plasmid.

When the insects ingest the inactive form of the toxin crystals or the protoxin, it enters the gut environment of the insect. Inside the gut due to the alkaline nature due to the alkaline pH of the gut, the inactive crystals denature and it becomes soluble. They react with the protease of the gut which causes the toxin to be released from the crystals. It then converts into an active form of the toxin.

The activated toxin then enters into the cells of the surface of the midgut and further it causes paralysis of the digestive tract. This reaction causes a pore inside the midgut epithelial cells resulting in swelling and finally lysis of the cell. Some of the bacteria may colonize the gut of the insect. The insect finally ceases to eat and eventually starves to death.

Hence, the correct answer is option (c).

Additional information:
The toxin crystals produced by Bacillus thuringiensis bear some specific types of activities against the insect species of the orders Lepidoptera, Diptera, Coleoptera and species of Hymenoptera. It also works as a potent toxin against the nematodes. It serves as an important reservoir of the Cry toxin which is used for the production of biological insecticides and pesticides.

Note: Bt toxins are also known as crystal proteins or Cry toxins and are potential toxins that are used as biopesticides against a variety of insects. The Bt toxin is produced by the bacterium Bacillus thuringiensis through the process of sporulation. The inactive form of the toxin upon ingestion by the insect moves to the alkaline pH containing environment which causes the toxin to change into active form.