Crystals of Bt toxin produced by some bacteria do not kill the bacteria themselves. Why?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Toxin is inactive within the bacterial cells.
Bacillus thuringiensis forms protein crystals that contain an insecticidal protein, the Bt toxin. Inside the bacterium the toxin is an inactive protoxin. It becomes active only in the alkaline gut of an insect, which dissolves the crystals. The active toxin then makes pores in the midgut cells and kills the insect.
Option (1) Bacteria are resistant to the toxin. — Incorrect. The bacterium is not protected by any resistance. It is safe because the toxin never becomes active inside it.
Option (2) Toxin is immature. — Incorrect. The book does not describe the toxin as immature. The correct word is inactive: it is a protoxin that needs the insect’s alkaline gut to become active.
Option (3) Toxin is inactive within the bacterial cells. — Correct. The Bt toxin exists as an inactive protoxin in the bacterial cell. Only the alkaline pH of the insect gut turns it into the active toxin, so the bacterium is not harmed.
Option (4) Bacteria enclose toxin in a special sac. — Incorrect. There is no special sac. The toxin lies in protein crystals, and it is harmless there because it is inactive.
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.”