What accident led Alexander Fleming to discover penicillin in 1928?

Answer

Fleming noticed a Penicillium mold had cleared a ring in a Staphylococcus culture. It became the first widely used antibiotic after later work by others.

Context

In September 1928 Fleming returned to St Mary's Hospital in London from a holiday and found a culture plate of Staphylococcus bacteria that had been contaminated by a blue-green mold. The bacteria near the mold had been destroyed. Fleming identified the mold as a Penicillium, grew it in broth and found that the liquid it produced, which he called penicillin, inhibited many disease-causing bacteria. He published the finding in 1929.

The story has more nuance than the popular version. Fleming was an experienced bacteriologist who had earlier discovered lysozyme, and he recognized what was unusual. But he found penicillin hard to purify and stabilize, and did not develop it into a drug. The work of turning it into a medicine was done a decade later at Oxford by Howard Florey, Ernst Chain and colleagues, with major production help from American laboratories during World War II. Fleming, Florey and Chain shared the 1945 Nobel Prize in Physiology or Medicine.

Historians also note earlier observations of mold antagonism toward bacteria, including by Ernest Duchesne in 1897 and others, but those did not lead to widely used treatments. Penicillin works by interfering with construction of bacterial cell walls, which human cells lack, and its spread led to the later emergence of resistant bacteria, a concern Fleming himself raised in his Nobel lecture.