Bacillus thuringiensis forms protein crystals which contain insectidical protein. This protein

Options

A. binds with epithelial cells of midgut of the insect pest ultimately killing it
B. is activated by acidic pH of the gut of the insect pest
C. does not kill the carrier bacterium which is itself resistant to this toxin
D. all of these

Answer

### Step-by-Step Solution:

1. **Understanding Bacillus thuringiensis (Bt)**:
- Bacillus thuringiensis is a soil bacterium known for producing protein crystals that contain insecticidal proteins, specifically known as Bt toxins.

2. **Function of Bt Toxins**:
- These proteins are effective against certain insect pests, including Lepidopterans (moths and butterflies), Dipterans (flies), and Coleopterans (beetles).

3. **Activation of Bt Toxins**:
- The Bt toxin exists in an inactive form when produced. It becomes active only after being ingested by an insect.

4. **Role of Insect Gut Environment**:
- Upon ingestion, the alkaline pH of the insect's gut activates the Bt toxin. This is a crucial step as the activation does not occur in an acidic environment.

5. **Mechanism of Action**:
- Once activated, the toxin binds to the epithelial cells of the midgut lining of the insect. This interaction leads to cell swelling and ultimately results in the death of the insect.

6. **Effect on the Carrier Bacterium**:
- Importantly, the Bt toxin does not harm the bacterium itself. Bacillus thuringiensis is resistant to its own toxin, allowing it to survive while targeting the insect pests.

7. **Evaluating the Options**:
- Option 1 states that the protein binds with the epithelial cells of the midgut, which is correct.
- Option 2 incorrectly states that the toxin is activated by acidic pH, which is false; it is activated by alkaline pH.
- Option 3 correctly states that the toxin does not kill the carrier bacterium, which is true.
- Therefore, options 1 and 3 are correct, while option 2 is incorrect.

8. **Conclusion**:
- The correct answers are options 1 and 3, as they accurately describe the action of the Bt toxin and its effect on the carrier bacterium.