Fleming, Sir Alexander (1881–1955), the British bacteriologist who discovered penicillin. One day in 1928 he found that a spore of mold had drifted into a colony of bacteria he was growing, and noticed that no bacteria were growing near the mold. After a series of tests, Fleming reported in 1929 that the mold contained a substance he called penicillin, capable of destroying bacteria without producing a toxic effect on the animal system. The discovery was ignored until 1938, when Howard W. Florey and Ernst B. Chain began to develop penicillin for medical use. The three scientists shared the 1945 Nobel Prize in physiology or medicine.

Fleming was born on a farm in Ayrshire, Scotland. He graduated from St. Mary's Hospital Medical School, affiliated with the University of London. He taught at St. Mary's until 1948, becoming a professor in 1928, and was knighted in 1944. Fleming in 1922 discovered lysozyme, an antiseptic substance found in the whites of eggs and in human saliva, nasal secretions, and tears.