Who discovered penicillin, the first true antibiotic?

• A. Alexander Fleming
• B. Louis Pasteur
• C. Robert Koch
• D. Ignaz Semmelweis

Answer: (A)

The essential idea is recognizing how an antibiotic was first found in nature and then developed into a practical medicine. Alexander Fleming noticed that a mold called Penicillium notatum had contaminated one of his Petri dishes with Staphylococcus bacteria, and around the mold the bacterial growth stopped. That clear zone of inhibition showed the mold was releasing a substance that killed or slowed bacteria, which he identified as penicillin. This accidental observation in 1928 revealed the existence of a true antibiotic, and it eventually led to the development of methods to purify penicillin and mass-produce it for medical use, especially after the crucial work of Florey and Chain in the 1940s. The discovery revolutionized treatment of bacterial infections and reshaped medicine, earning Fleming, Florey, and Chain a Nobel Prize in 1945.

The other scientists mentioned contributed in different, foundational ways: Pasteur advanced germ theory and practices to control microorganisms; Koch established how to link specific microbes to diseases; Semmelweis championed handwashing and antiseptic technique to prevent infections in medical settings.