Sir Alexander Fleming (1881-1955), British bacteriologist and Nobel laureate, best known for his discovery of penicillin. Born near Darvel, Scotland, and educated at Saint Mary's Hospital Medical School of the University of London, he served as professor of bacteriology at St. Mary's Hospital Medical School from 1928 to 1948, when he became professor emeritus.

Fleming conducted outstanding research in bacteriology, chemotherapy, and immunology. In 1922 he discovered lysozyme, an antiseptic found in tears, body secretions, albumen, and certain fish plants. His discovery of penicillin came about accidentally in 1928 in the course of research on influenza. His observation that the mold contaminating one of his culture plates had destroyed the bacteria laid the basis for the development of penicillin therapy (see Antibiotic).

Fleming was knighted in 1944. In 1945 he shared the Nobel Prize in physiology or medicine with Australian scientist Howard Walter Florey and German-British pathologist Ernst Boris Chain for their contributions to the development of penicillin.