A moldy petri dish in 1928 — and the antibiotic age began.
Alexander Fleming (1881–1955) was the Scottish bacteriologist who discovered penicillin in 1928, when a Penicillium mold drifted onto a staphylococcus culture and killed the bacteria around it — the chance observation that launched the antibiotic revolution and earned him the 1945 Nobel Prize.
Working at St Mary's Hospital in London, he noticed what others might have thrown away: a clear ring where bacteria could not grow. He isolated the mold's active substance, named it penicillin, and published — then watched the world ignore it for a decade.
Fleming was born in 1881 on a farm in Ayrshire, Scotland, and trained at St Mary's Hospital Medical School in London, where he stayed for his whole career. As a young doctor in the First World War he saw soldiers die of infected wounds that antiseptics could not save — the experience that set his life's question.
In 1922 he had already discovered lysozyme, an antibacterial enzyme found in tears and mucus. But his laboratory was famously untidy — and in September 1928, returning from a holiday, he found the contaminated plate that changed medicine.
Fleming showed that the mold's juice killed a wide range of dangerous bacteria while sparing white blood cells. He published his findings in 1929 — and almost nothing happened. Penicillin was unstable, hard to purify, and Fleming, no chemist, moved on.
A decade later at Oxford, Howard Florey, Ernst Chain and Norman Heatley took up the problem, purified penicillin, and proved it in animals and patients. By D-Day in 1944, Allied factories were making enough to treat wounded soldiers by the hundred thousand; after the war it reached the world.
In 1945 Fleming shared the Nobel Prize in Physiology or Medicine with Florey and Chain. In his Nobel lecture he issued a warning that reads like prophecy: misuse of penicillin would breed resistant bacteria — 'the thoughtless person playing with penicillin treatment is morally responsible for the death of the man who finally succumbs to infection.'
Antibiotic resistance is now one of the great medical challenges of the century — the shadow side of the miracle Fleming found in a dirty dish. He died in London in 1955 and is buried in St Paul's Cathedral.
