Penicillin (Fleming's Discovery)

Observation that a Penicillium mold contaminating a bacterial culture plate produced a substance that lysed/inhibited nearby Staphylococcus colonies -- a natural chemical-warfare trait of the mold against competing microbes. Alexander Fleming, "On the Antibacterial Action of Cultures of a Penicillium, with Special Reference to their Use in the Isolation of B. influenzae" (Br. J. Exp. Pathol. 10, 226-236, published 1929-06-01, OpenAlex W2177846120, 1,570 citations) -- the actual publication date, not the "1928" observation date commonly cited in secondary sources. Fleming did not purify or mass-produce it; Florey, Chain and their Oxford team achieved that in the early 1940s, enabling wartime mass production. Fleming, Florey and Chain shared the 1945 Nobel Prize in Physiology or Medicine. Founded the entire antibiotic drug class, predating CRISPR by roughly 80 years as a translation of a microbial defense mechanism into an industry.