Sir Alexander Fleming FRS FRSE FRCS (6 August 1881 – 11 March 1955) was a Scottish physician and microbiologist, best known for discovering the world's first broadly effective antibiotic substance, which he named penicillin.

He earned his MB, BS, (London), with a Gold Medal in 1908, and became a lecturer at St. Mary's until 1914. He served during World War I as a captain in the Army Medical Corps, mentioned in dispatches, and in 1918 he returned to St. Mary's. He was elected Professor of the School in 1928 and Emeritus Professor of Microbiology, University of London in 1948.

Early in his medical life, Fleming became interested in the natural bacterial action of the blood and in antiseptics. He was able to continue his studies throughout his military career and on demobilization he settled to work on antibacterial substances which would not be toxic to animal tissues. In 1921, he discovered in «tissues and secretions» an important bacteriolytic substance which he named Lysozyme. About this time, he devised sensitivity titration methods and assays in human blood and other body fluids, which he subsequently used for the titration of penicillin.

In 1928, while working on influenza virus, he observed that mould had developed accidently on a staphylococcus culture plate and that the mould had created a bacteria-free circle around itself. He was inspired to further experiment and he found that a mould culture prevented growth of staphylococci, even when diluted 800 times. He named the active substance penicillin.

Penicillin is the first antibiotic and its true potential came to life in 1940 and wide fermentation practice was developed for the production of antibiotics. Staphylococci were the human infection, which got cured by the first penicillin.

Sir Alexander wrote numerous papers on bacteriology, immunology and chemotherapy, including original descriptions of lysozyme and penicillin. They have been published in medical and scientific journals.

His discovery of penicillin changed the world of modern medicine by introducing the age of useful antibiotics; penicillin has saved, and is still saving, millions of people around the world.

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