Who discovered penicillin, the first antibiotic?

• A. Alexander Fleming
• B. Louis Pasteur
• C. Robert Koch
• D. John Snow

Answer: (A)

Penicillin was found because a chance observation revealed that a mold growing on a bacterial culture released a substance that stopped the surrounding bacteria from growing. Alexander Fleming noticed a halo of clearing around the mold in a Staphylococcus plate, identified the mold as Penicillium notatum, and concluded it produced an antibacterial compound he named penicillin. This simple, once-hidden observation opened the era of antibiotics, transforming medicine by enabling treatment of bacterial infections. While Fleming didn’t purify or mass-produce penicillin himself, his record as the discoverer is the reason the antibiotic era began. The other scientists listed made major contributions to microbiology in different ways—Pasteur advanced germ theory and vaccines, Koch linked microbes to disease, Snow studied how cholera spreads—yet none discovered penicillin.