Sir Alexander Fleming (1881-1955) was a Scottish biologist and pharmacologist. He wrote many articles on bacteriology, immunology, and chemotherapy. His best-known discoveries are the discovery of the enzyme lysozyme in 1923 and the antibiotic substance penicillin from the mold Penicillium notatum in 1928, for which he shared the Nobel Prize in Physiology or Medicine in 1945 with Howard Florey and Ernst Boris Chain.