Many know the story of Alexander Fleming’s chance discovery of penicillin. Fleming, a bit of an absent-minded professor (and a bit of a slob), left culture plates streaked with Staphylococcus on his lab bench while he went away on summer holiday. When he returned, he found that “a mould” had contaminated one of his plates, probably having floated in from an open window. Before discarding the plate, he noticed that, within a “ring of death” around the mold, the bacteriahad disappeared. Something in the “mould juice” had killed the staphylococci.

Fleming immediately began investigating this strange new substance. He identified the mold as Penicillium rubrum and named the substance penicillin.¹ He publishedhis findings in the spring of 1929 in The British Journal of Experimental Pathology.²But a decade later, pharmacologist Howard Florey and biochemist Ernst Chain at Oxford would pick up where Fleming left off. Alongside a USDA lab in Peoria, Illinois, the pair would develop penicillin into a life-saving drug and usher in the era of antibiotics.

This is the kind of science story everyone likes. One of serendipity and accidental discovery; a chance observation that changed the world. But is it true?

For decades, scientists and historians have puzzled over inconsistencies in Fleming’s story. For starters, the window to Fleming’s lab was rarely (if ever) left open, precisely to prevent the kind of contamination that supposedly led to penicillin’s discovery. Second, the story is strikingly similar to Fleming’s earlier discovery of lysozyme, another antibacterial substance, which also featured lucky contamination from an open window. Third, Fleming claimed to have discovered the historic culture plate on September 3^rd, but the first entry in his lab notebook isn’t dated until October 30^th, nearly two months later.

Last, and most important: penicillin only works if it’s present before the staphylococci. Fleming did not know it at the time, but penicillin interferes with bacterial cell wall synthesis, which only happens when bacteria are actively growing. Visible colonies, however, are composed mostly of mature or dead cells. By the time a colony can be seen, it is often too late for penicillin to have any effect. In fact, the Penicillium mold typically won’t even grow on a plate already filled with staphylococcus colonies. For years, scientists have attempted to replicate Fleming’s original discovery. All have met with failure.

Thus, it’s difficult to reconcile Fleming’s story with these historical and scientific discrepancies. Did he misremember events from 15 years earlier? Could he have fudged the details to make for a more compelling narrative? Or, might Fleming’s experiment have been subject to an unusual confluence of chance events unbeknownst even to him?

Speculation about how Fleming discovered penicillin is of little consequence compared to its practical impact. However, science is about evaluating evidence and moving closer to the “truth.” As we near the 100th anniversary of penicillin’s discovery — which undoubtedly will encourage even greater repetition of the story — it’s in this spirit that we must scrutinize the story’s veracity.

The historical and scientific data are limited and often contradictory. Nevertheless, several scientists and historians have worked hard to piece together what facts are certain and fill the gaps with their most probable guesses. The result is a range of competing theories, each attempting to explain what really happened in that St. Mary’s Hospital laboratory in the summer of 1928.

Read the full article by Kevin S. Blake at Asimov Press.