All the bacteria fix nitrogen except

Options

A. Rhizobium
B. E. coli
C. Azotobacter
D. Cyanobacteria

Explanation

To solve the question "All the bacteria fix nitrogen except," we need to identify which of the given options does not have the capability to fix nitrogen. Here's a step-by-step solution:

### Step 1: Understand Nitrogen Fixation
- Nitrogen fixation is the process by which certain microorganisms convert atmospheric nitrogen (N₂) into a form that can be used by living organisms, such as ammonia (NH₃).

### Step 2: Identify Nitrogen-Fixing Bacteria
- Common nitrogen-fixing bacteria include:
- **Rhizobium**: These bacteria form symbiotic relationships with legumes and are well-known for fixing nitrogen.
- **Azotobacter**: A free-living nitrogen-fixing bacterium found in soil.
- **Cyanobacteria**: Also known as blue-green algae, these can fix nitrogen in aquatic environments and soil.

### Step 3: Examine the Options
- The options provided are:
1. Rhizobium
2. E. coli
3. Azotobacter
4. Cyanobacteria

### Step 4: Identify the Non-Nitrogen Fixing Bacterium
- **E. coli** is a type of bacteria that primarily resides in the intestines of humans and animals. It is not known for fixing nitrogen. Instead, it plays a role in digestion and can be harmless or pathogenic.

### Step 5: Conclusion
- Therefore, the bacterium that does not fix nitrogen among the options given is **E. coli**.

### Final Answer
- All the bacteria fix nitrogen except **E. coli**.

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