Q.When Bacillus thuringiensis enters a certain insect’s body, the insect gets killed, but itself remains unaffected. Explain how it is possible.
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Start your 14-day free trial to unlock the full solution →The bacterium Bacillus thuringiensis (Bt) produces a toxin that is harmless to itself because it stores the toxin in an inactive form, which only becomes active inside the alkaline gut of specific insects.
To understand this, you first need to appreciate the bacterium’s strategy. Bacillus thuringiensis is a soil-dwelling bacterium that has evolved a remarkable weapon: it produces a protein crystal, called the Bt toxin, during a particular phase of its growth. This crystal is not a poison in the usual sense — it is a protoxin, meaning it is completely harmless to the bacterium itself. The bacterium keeps this toxin locked in an inactive, safe form inside its own cells. It does not need to protect itself from its own weapon because the weapon is stored as a harmless precursor.
The magic — and the specificity — happens only when a susceptible insect ingests the bacterium (or the crystal protein). The insect’s gut is the key. Most insects that Bt targets have an alkaline gut (high pH), which is very different from the neutral or acidic environment inside the bacterium or in most other organisms. When the inactive protoxin enters this alkaline environment, it dissolves and gets cleaved by specific proteases (enzymes) present in the insect’s gut. This cleavage converts the harmless protoxin into an active, toxic form.
The activation is highly specific. The alkaline pH and the particular proteases found in the guts of certain insects (like caterpillars, beetles, and mosquitoes) are what trigger the toxin. Mammals, including humans, have acidic stomachs and different digestive enzymes, so the Bt protoxin passes through our system without ever becoming active. This is why Bt is considered safe for us and is widely used in organic farming.
Once activated, the toxin binds to specific receptor proteins on the lining of the insect’s midgut. These receptors act like a lock, and the activated toxin is the key. The binding causes the toxin to insert itself into the cell membrane of the gut cells, forming tiny pores. These pores disrupt the cell’s ability to regulate the movement of ions and water. The cells swell, burst, and the gut lining is destroyed. The insect stops feeding, the gut contents leak into its body cavity, and it dies — usually within a couple of days. …
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