Biography: Many famous scientific discoveries come about by accident, and such was the case with penicillin. The first and still best-known antibiotic, penicillin is a natural substance excreted by a type of mold of the genus Penicillium. It so happened that a Scottish bacteriologist, Alexander Fleming, was doing research on staphylococcal bacteria in the late 1920s and noticed that one culture had become contaminated with some mold. What was curious was that there was a circular area around the mold that was free of bacterial growth. After some investigation, Fleming discerned that the mold was excreting a substance deadly to the bacteria, and he named it penicillin in the mold's honor. Fleming had already discovered another natural antibacterial substance a few years earlier in 1921-lysozyme, an enzyme contained in tears and saliva. But the discovery of penicillin was of far greater importance, although its impact was not fully felt right away because Fleming lacked the equipment necessary to isolate the active compound and to synthesize it in quantities that could be used medicinally. This happened a dozen years later during World War II and stimulated the development of new drugs that could fight infections transmitted on the battlefield. Two other scientists, Ernst Chain and Howard Florey, were responsible for this further work, and together with Fleming the three shared the 1945 Nobel Prize for physiology or medicine.