The most common bacterium used in genetic engineering is:

Options

A. E. coli
B. Clostridium
C. Bacillus thuringiensis
D. Salmonella

Explanation

**Step-by-Step Solution:**

1. **Identify the Question**: The question asks for the most common bacterium used in genetic engineering.

2. **List the Options**: The options provided are:
- E. coli
- Clostridium
- Bacillus thuringiensis
- Salmonella

3. **Analyze Each Option**:
- **E. coli**: This bacterium is widely used in genetic engineering due to its rapid growth rate, ease of manipulation, and ability to express proteins at high levels.
- **Clostridium**: While it has its uses, it is not the most common choice for genetic engineering.
- **Bacillus thuringiensis**: This bacterium is primarily known for its insecticidal properties and is not the most common in genetic engineering.
- **Salmonella**: This bacterium is generally associated with foodborne illnesses and is not commonly used in genetic engineering.

4. **Select the Correct Answer**: Based on the analysis, **E. coli** is the most common bacterium used in genetic engineering.

5. **Justify the Answer**: E. coli is preferred because:
- It serves as an efficient host for gene cloning.
- It allows for the easy introduction of DNA molecules.
- It has a rapid growth rate, leading to quicker results.
- It can express proteins at very high levels.

6. **Final Answer**: The most common bacterium used in genetic engineering is **E. coli**.

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