__________is a source of antibiotic.

Explanation

**Step-by-Step Solution:**

1. **Understanding Antibiotics**: Antibiotics are substances that kill or inhibit the growth of microorganisms, particularly bacteria and fungi. They are crucial in treating infections caused by these pathogens.

2. **Sources of Antibiotics**: Antibiotics are primarily derived from microorganisms, specifically certain types of bacteria and fungi. This means that the natural world, particularly the microbial world, is a rich source of these medicinal compounds.

3. **Historical Context**: The first antibiotic discovered was penicillin, which was identified by Alexander Fleming in 1929. This discovery marked a significant advancement in medical science and the treatment of infectious diseases.

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