Which one of the following is a free-living obligate anaerobic bacterium?

Options

A. (a) Clostridium
B. (b) Rhodospirillum
C. (c) Azotobacter
D. (d) Bacillus subtilis

Explanation

To determine which of the following is a free-living obligate anaerobic bacterium, we need to analyze the options provided:

1. **Understanding Obligate Anaerobic Bacteria**:
- **Definition**: Obligate anaerobic bacteria are microorganisms that cannot survive in the presence of oxygen. They require an environment devoid of oxygen to grow and thrive.
- **Characteristics**: These bacteria are harmed or inhibited by even the smallest amounts of oxygen.

2. **Analyzing the Options**:
- **Option A: Clostridium**: This genus is known for being obligate anaerobes. They thrive in environments without oxygen and are often associated with food poisoning and spoilage of canned foods.
- **Option B: Rhodospirillum**: This is a genus of bacteria that can be photosynthetic and can tolerate oxygen, thus not fitting the definition of obligate anaerobes.
- **Option C: Azotobacter**: This genus is known to be free-living nitrogen-fixing bacteria that can tolerate oxygen, making them facultative anaerobes rather than obligate anaerobes.
- **Option D: Bacillus subtilis**: This bacterium is a facultative anaerobe, meaning it can survive with or without oxygen.

3. **Conclusion**:
- Based on the definitions and characteristics, the only option that fits the criteria of being a free-living obligate anaerobic bacterium is **Option A: Clostridium**.

### Final Statement:
Hence, the correct answer is **Option A: Clostridium**.

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