Fleming discovered penicillin in a moldy dish

On 28 September 1928, bacteriologist Alexander Fleming returned to his lab at St Mary’s Hospital in London and began sorting through Petri dishes of Staphylococcus he had left out before a holiday. On one plate a blue-green mold had taken hold, and around it the bacteria had simply dissolved away.

The intruder was a Penicillium mold, and Fleming realized it was releasing some substance lethal to the bacteria. He called the active liquid penicillin.

When I woke up just after dawn on September 28, 1928, I certainly didn’t plan to revolutionize all medicine by discovering the world’s first antibiotic.

Fleming published his findings, but he could not purify the compound at scale, and the work languished for a decade. Only in the early 1940s did Howard Florey and Ernst Chain turn it into a usable drug, work that earned the three men the 1945 Nobel Prize. A chance contamination had opened the antibiotic era.