Alexander Fleming was a Scottish physician and microbiologist who is best known for his discovery of the antibiotic penicillin. In 1928, while working at St. Mary's Hospital in London, Fleming discovered that a mold called Penicillium notatum had contaminated one of his petri dishes and was preventing the growth of bacteria. He realized that the mold produced a substance that could kill bacteria, and named it penicillin.This discovery led to the development of penicillin as a widely-used antibiotic to treat bacterial infections. Penicillin has been credited with saving countless lives and revolutionizing medicine. Fleming was awarded the Nobel Prize in Physiology or Medicine in 1945 for his discovery of penicillin.