American Scientist

Alexander Fleming was a Scottish bacteriologist who possessed a streak of play. Inside his London laboratory, he mostly thought about mechanisms to stave off illnesses caused by germs. Usually, he smeared straight lines of bacteria onto agar-coated petri dishes during his experiments. But sometimes Fleming painted pictures with them, and even left the dishes out for extended periods. On one September day in 1928, Fleming was busily cleaning petri dishes that he had left before his summer vacation, and he noticed something in one of them. Colonies of staphylococci were succumbing in locations next to a mold that had contaminated the petri dish as it sat during his time away. Curious, Fleming captured part of the mold and grew it in a broth. A white, fluffy mass grew on top of the broth, followed by a dark green felt. A few days later, the broth turned a bright yellow. Fleming identified the mold as a member of the Penicillium genus, and he believed that the mold was secreting some antibacterial substance into the broth. He filtered the mold’s broth and called it “penicillin.”