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Alexander Fleming Discovers Penicillin

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Picture of Sir Alexander Fleming, who discovered penicillin.

British bacteriologist and Nobel laureate Sir Alexander Fleming (1881 - 1955) in his laboratory at St Mary's Hospital, Paddington. (1941)

(Photo by Topical Press Agency/Getty Images)

In 1928, bacteriologist Alexander Fleming made a chance discovery from an already discarded, contaminated Petri dish. The mold that had contaminated the experiment turned out to contain a powerful antibiotic, penicillin. However, though Fleming was credited with the discovery, it was over a decade before someone else turned penicillin into the miracle drug for the 20th century.

How did this Petri dish almost get cleaned before being noticed? How did the mold get onto the dish? Who transformed penicillin into a useful drug?

The Chance Discovery

On a September morning in 1928, Alexander Fleming sat at his work bench at St. Mary's Hospital after having just returned from a vacation at The Dhoon (his country house) with his family. Before he had left on vacation, Fleming had piled a number of his Petri dishes to the side of the bench so that Stuart R. Craddock could use his work bench while he was away.

Back from vacation, Fleming was sorting through the long unattended stacks to determine which ones could be salvaged. Many of the dishes had been contaminated. Fleming placed each of these in an ever growing pile in a tray of Lysol.

Much of Fleming's work focused on the search for a "wonder drug." Though the concept of bacteria had been around since Antonie van Leeuwenhoek first described it in 1683, it wasn't until the late nineteenth century that Louis Pasteur confirmed that bacteria caused diseases. However, though they had this knowledge, no one had yet been able to find a chemical that would kill harmful bacteria but also not harm the human body.

In 1922, Fleming made an important discovery, lysozyme. While working with some bacteria, Fleming's nose leaked, dropping some mucus onto the dish. The bacteria disappeared. Fleming had discovered a natural substance found in tears and nasal mucus that helps the body fight germs. Fleming now realized the possibility of finding a substance that could kill bacteria but not adversely affect the human body.

In 1928, while sorting through his pile of dishes, Fleming's former lab assistant, D. Merlin Pryce stopped by to visit with Fleming. Fleming took this opportunity to gripe about the amount of extra work he had to do since Pryce had transferred from his lab. To demonstrate, Fleming rummaged through the large pile of plates he had placed in the Lysol tray and pulled out several that had remained safely above the Lysol. Had there not been so many, each would have been submerged in Lysol, killing the bacteria to make the plates safe to clean and then reuse.

While picking up one particular dish to show Pryce, Fleming noticed something strange about it. While he had been away, a mold had grown on the dish. That in itself was not strange. However, this particular mold seemed to have killed the Staphylococcus aureus that had been growing in the dish. Fleming realized that this mold had potential.

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