Alexander Fleming won the Nobel Prize for the discovery of what?

Alexander Fleming won the Nobel Prize for the discovery of penicillin. He shared the 1945 Nobel Prize in Physiology or Medicine with Ernst Boris Chain and Howard Walter Florey, who were honored for penicillin’s discovery and its curative effects against infectious diseases.

Fleming noticed the antibacterial effect in 1928 after a mold contaminated one of his laboratory cultures of Staphylococcus bacteria. The mold, later identified as Penicillium notatum, prevented bacteria from growing around it. Fleming named the active substance penicillin, but extracting and producing it reliably was initially difficult.

Researchers at Oxford, especially Chain and Florey, later developed methods to purify and test penicillin. Large-scale production during World War II transformed it into a practical medicine. Fleming is often described as the sole discoverer, but the Nobel recognized both the original observation and the later work that made treatment possible. Penicillin also helped launch the antibiotic era, although Fleming warned that misuse could encourage bacterial resistance.