List two bacteria which bring about nitrification.

Explanation

**Step-by-Step Solution:**

1. **Understanding Nitrification**: Nitrification is a biological process in the nitrogen cycle where ammonia is converted into nitrites and then into nitrates. This process is crucial for soil fertility and plant growth.

2. **Identifying the Two Steps of Nitrification**:
- The first step involves the conversion of ammonia (NH₃) into nitrites (NO₂⁻).
- The second step involves the conversion of nitrites (NO₂⁻) into nitrates (NO₃⁻).

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