How did a mouldy dish give us the first antibiotic?

Penicillin was discovered by accident, thanks to a messy bench and a family holiday. In 1928, Alexander Fleming left dishes of staphylococcus bacteria in a corner of his lab and went away with his family. When he came back on 3 September, one dish had been contaminated by a mould. Around the mould, the bacteria had been destroyed, while colonies farther away were fine. Fleming's famous reaction was: "That's funny."

The mould was releasing a substance that killed bacteria. After months of calling it "mould juice", Fleming named it penicillin in 1929. We now know how it works: penicillin blocks the enzymes bacteria use to stitch together their cell walls. The wall weakens, water floods in, and the cell bursts.

But Fleming couldn't turn his mould juice into a medicine. For about twelve years he kept the mould alive and looked for a chemist who could purify it. The breakthrough came in Oxford, where Howard Florey and Ernst Chain purified penicillin and, with wartime support from Britain and the United States, worked on mass-producing it. When Fleming announced he'd visit the Oxford team in 1940, Chain was startled: "Good God! I thought he was dead." In 1945 the three men shared the Nobel Prize.