Which of the following triggers the activation of the toxin of Bacill…

The correct answer is Option (1) → Alkaline pH of gut

Bacillus thuringiensis makes protein crystals that hold an insecticidal toxin. The toxin does not kill the bacterium itself because it is made as an inactive protoxin. Once an insect eats it, the alkaline pH of the insect’s gut dissolves the crystals and turns the protoxin into the active toxin. The active toxin binds to the midgut cells and makes pores in them, so the cells swell, burst and the insect dies.

Option (1) Alkaline pH of gut — Correct. The alkaline pH of the gut solubilises the crystals and activates the toxin.

Option (2) Acidic pH of stomach — Incorrect. The toxin is activated in an alkaline gut, not an acidic one. An acidic pH would not dissolve the crystals in this way.

Option (3) A mechanical action in insect gut — Incorrect. Activation is a chemical change brought about by pH. Grinding or any other mechanical action in the gut does not activate the protoxin.

Option (4) High temperature — Incorrect. Heat has no part in activating the Bt toxin. The change takes place at the pH of the insect gut.

NCERT Extract:
Biotechnological Applications in Agriculture – “… B. thuringiensis forms protein crystals during a particular phase of their growth. These crystals contain a toxic insecticidal protein. … Actually, the Bt toxin protein exist as inactive protoxins but once an insect ingest the inactive toxin, it is converted into an active form of toxin due to the alkaline pH of the gut which solubilise the crystals. The activated toxin binds to the surface of midgut epithelial cells and create pores that cause cell swelling and lysis and eventually cause death of the insect.”