Biology · Biotechnology and Its Applications · NEET
Bt toxin does not kill Bacillus thuringiensis because inside the bacterium the toxin is stored as an inactive protoxin, packed into crystal form. It becomes an active, cell-killing toxin only when an insect eats it and the alkaline pH of the insect gut solubilises the crystal. Memory hook: "Sleeping inside the bug-maker, awake only in the bug's alkaline stomach."
Bt toxin stays as inactive protoxin crystal inside the bacterium (harmless), and only the alkaline pH of the insect gut converts it to active toxin that kills the insect.
Your doubts, answered
Why does the Bt toxin not kill the bacterium that produces it?
Because inside Bacillus thuringiensis the toxin is present in an inactive form called a protoxin. An inactive protoxin cannot form pores in cells, so it does no harm to the bacterium. It is only converted into the active toxin later, inside the insect gut.
What is the difference between protoxin and active toxin here?
Protoxin is the inactive, stored form found inside the bacterium as a crystal. Active toxin is the working form that binds midgut cells and makes pores. The change from protoxin to active toxin needs the alkaline pH of the insect gut.
What triggers the protoxin to become active?
The alkaline pH of the insect gut. This alkaline pH dissolves (solubilises) the protein crystals and turns the inactive protoxin into the active toxin. This is a very common NEET one-mark point.
Why is the toxin kept as a crystal inside the bacterium?
The crystal is a solid, inactive protoxin store made during a particular growth phase. Being inactive and crystalline, it stays harmless to the bacterium until it is dissolved by the alkaline gut of an insect that eats it.
What happens to the insect after the toxin becomes active?
The active toxin binds the surface of midgut epithelial cells and creates pores. This causes the cells to swell and burst (lysis), and the insect eventually dies. This is why Bt is used as a natural insecticide.
Does human stomach activate the Bt toxin?
No. The human stomach is acidic, not alkaline, so it does not convert the protoxin into active toxin. This gut-pH difference is one reason Bt is considered specific to certain insects and not harmful to humans in the same way.
⚠️ The NEET trap ✗ The Bt toxin is activated by the acidic pH of the insect gut. ✓ The Bt toxin is activated by the ALKALINE pH of the insect gut, which solubilises the crystals. 🧠 NEET 2024 flipped this into a true/false statement question. Alkaline gut = active toxin. Acidic = wrong. The stomach of the insect midgut is alkaline, not acidic.
Real NEET questions
NEET 2019
What triggers activation of protoxin to active Bt toxin of Bacillus thuringiensis in boll worm?
A · Body temperature
B · Moist surface of midgut
C · Alkaline pH of gut ✓
D · Acidic pH of stomach
Solution: Inside the bacterium the toxin stays as an inactive protoxin crystal, so it does not kill Bacillus. When the bollworm eats it, the alkaline pH of the insect gut solubilises the crystals and converts the protoxin into active toxin, which then makes pores in midgut cells.
NEET 2024
Statement I: Bt toxins are insect group specific and coded by a gene cry1Ac. Statement II: Bt toxin exists as inactive protoxin in B. thuringiensis. However, after ingestion by the insect the inactive protoxin gets converted into active form due to acidic pH of the insect gut. Choose the correct answer.
A · Both Statement I and Statement II are false
B · Statement I is true but Statement II is false ✓
C · Statement I is false but Statement II is true
D · Both Statement I and Statement II are true
Solution: Statement I is true: Bt toxins are insect-group specific and coded by cry genes such as cry1Ac. Statement II is FALSE only because it says acidic pH. The correct fact is that the inactive protoxin is activated by the ALKALINE pH of the insect gut. The part that the toxin exists as inactive protoxin inside the bacterium is itself correct.
Solved Biotechnology and Its Applications NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
In what form is the Bt toxin present inside Bacillus thuringiensis?
As an inactive protoxin packed into protein crystals, formed during a particular phase of the bacterium's growth. Being inactive, it does not harm the bacterium.
What is the one-line NEET answer for why Bt toxin does not kill the bacterium?
Because the toxin exists as an inactive protoxin inside the bacterium; it is activated into toxic form only by the alkaline pH of the insect gut.
Which pH activates the Bt protoxin?
Alkaline pH of the insect gut. Remember: alkaline gut activates, acidic does not. NEET has repeatedly tested this exact point.
What does the active toxin do to the insect?
It binds midgut epithelial cells and creates pores, causing the cells to swell and lyse, leading to the death of the insect.
Is Bt toxin harmful to the plant carrying the cry gene?
No. The plant makes the protoxin form, which needs the specific alkaline gut of a target insect to become active, so it does not damage the plant's own cells.
