How did we discover penicillin, and how did it become a widespread, lifesaving medicine?
When Alexander Fleming, a Scottish physician and microbiologist, returned from a vacation in September 1928, an unexpected gift awaited him in the lab: mold that had grown in his absence. While observing petri dishes with colonies of Staphylococcus, a bacterium that can cause fatal infections, he noticed that no colonies were present around the mold growing on the dish (1). Fleming later identified the mold as belonging to the genus Penicillium, which is effective against Gram-positive bacteria that cause illnesses such as pneumonia, scarlet fever, and meningitis (2). The liquid secreted by the mold was further analyzed, and an early form of penicillin was identified. This discovery was revolutionary, but after this initial finding, challenges arose.
In the midst of World War II, mass production and research on Penicillin in the late 1930s and early 1940s proved to be difficult. Fleming was unable to isolate the active ingredient in the “mold juice,” and so the task was left to Dr. Howard Florey, a Pathology professor at Oxford (3). Alongside Ernst Chain and Norman Heatley, Florey researched ways to mass-produce penicillin. To supply their never-ending need for mold, the group grew it in baths, milk churns, and bedpans (1, 3). Despite their efforts, the demand for penicillin was too high. In early cases of penicillin use, the drug was effective in temporarily curing patients, but once supplies ran out, their conditions deteriorated (1, 3).
Because Britain could not keep up with demand for penicillin, production shifted to the United States in the early 1940s. Florey decided to work with U.S. entities, specifically the Department of Agriculture and the Rockefeller Foundation in New York. In exchange for his expertise, Florey would receive a kilogram of penicillin to continue trials and testing in Britain (2). Eventually, Andrew Moyer, a member of the Department of Agriculture, made two major discoveries that changed the production of penicillin. First, Moyer realized that lactose was a more efficient carbon and energy source than sucrose (1). Then, he discovered that corn could be used to produce much more penicillin than a typical batch. With these findings, submerged culture fermentation in tanks using corn steep liquor yielded penicillin batches ten times larger than what Florey and his team could produce in Britain (1).
With the ability to produce penicillin in larger quantities, Pfizer opened up the first commercial plant for large-scale penicillin production in the mid-1940s in Brooklyn, New York. Subsequently, on March 15, 1945, penicillin became widely available in pharmacies and to the American public. Restrictions on its availability, once in place because of limited quantities, were removed (1). Today, penicillin remains an important antibiotic used to treat deadly bacterial infections. Who knows where we would be today if Alexander Fleming never decided to take that vacation!
Bibliography:
- American Chemical Society International Historic Chemical Landmarks. Discovery and Development of Penicillin. http://www.acs.org/content/acs/en/education/whatischemistry/landmarks/flemingpenicillin.html (accessed Jan 19, 2026).
- Tan, S. Y., & Tatsumura, Y. (2015). Alexander Fleming (1881-1955): Discoverer of penicillin. Singapore medical journal, 56(7), 366–367. https://doi.org/10.11622/smedj.2015105
- Bernard, D. (2020, July 11). How a miracle drug changed the fight against infection during World War II, The Washington Post. https://www.washingtonpost.com/history/2020/07/11/penicillin-coronavirus-florey-wwii-infection/
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