For the first time, the bacteria were observed by

Options

A. Robert Koch
B. A.V. Leeuwenhoek
C. W.H. Stanley
D. Louis Pasteur

Answer

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks who first observed bacteria. This requires knowledge of historical figures in microbiology.

2. **Identifying the Options**: The options given are Robert Koch, A.V. Leeuwenhoek, W.H. Stanley, and Louis Pasteur. Each of these scientists contributed significantly to microbiology, but we need to identify who specifically observed bacteria first.

3. **Knowledge of Historical Context**:
- Robert Koch is known for his work on tuberculosis and the germ theory of disease.
- Louis Pasteur is famous for his work on pasteurization and vaccines.
- W.H. Stanley is recognized for his work on viruses, particularly tobacco mosaic virus.

4. **Focusing on A.V. Leeuwenhoek**: A.V. Leeuwenhoek, a Dutch scientist, is known for his pioneering work in microscopy. In the late 1670s, he was the first to observe and describe single-celled organisms, which included bacteria.

5. **Confirming the Observation**: In 1676, Leeuwenhoek used a simple microscope to observe bacteria, which he referred to as "animalcules." He documented his findings and communicated them to the Royal Society of London.

6. **Conclusion**: Based on the historical context and contributions of the scientists mentioned, the correct answer to the question is A.V. Leeuwenhoek.

**Final Answer**: For the first time, bacteria were observed by A.V. Leeuwenhoek in 1676.

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