Alexander Fleming discovered penicillin in 1928. He observed that a mold called Penicillium notatum contaminated a Staphylococcus culture and inhibited nearby bacterial growth, leading him to identify penicillin as the antibacterial substance produced by the mold. Fleming published his findings in 1929, and the antibiotic was later developed for medical use with key contributions from Howard Florey, Ernst Boris Chain, and Norman Heatley, culminating in their Nobel Prize in Physiology or Medicine in 1945.