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What Incredible Scientific Discovery Was Once Called “Mold Juice”?

Published Jun 18, 2024
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Some of history’s most incredible scientific breakthroughs have occurred in completely unexpected ways, leaving scientists unprepared to give their discoveries a proper name. One such example was the discovery of penicillin.

On September 28, 1928, upon returning to his lab at St. Mary’s Hospital in London after a vacation in Scotland, microbiologist Alexander Fleming discovered that the fungus Penicillium notatum had contaminated and destroyed some of the Staphylococcus bacteria he had been studying. Further investigation proved what Fleming had suspected: the Penicillium mold could kill bacteria and had the potential to cure deadly infections. It would ultimately become the world’s first antibiotic, but before naming it penicillin, he simply called it “mold juice.”

Fleming’s discovery would revolutionize the medical field, giving doctors a reliable tool to help save patients from diseases that would previously have been a death sentence. However, a great deal of research was required before this mold juice could be used on humans, and other scientists took up that task. At Oxford University, Dr. Howard Florey and his colleagues, most notably Dr. Ernst Chain, undertook experiments in the summer of 1940 to isolate and purify the active ingredient in mold juice and figure out how to use it. They studied 50 mice infected with streptococcus, injecting half with penicillin. Those mice survived, while the others died of sepsis.

The next step was producing penicillin at a scale large enough to provide effective treatment. An Oxford police constable who had been infected by streptococci and staphylococci while gardening was the first person treated by Florey and Chain’s “purified” penicillin, but they tragically did not have enough to cure him. Biochemist Norman Heatley also made major contributions to figuring out how to mass-produce penicillin. Working with scientists in Peoria, Illinois, in 1941, Florey and Heatley searched for a more productive fungus species to produce a reliable supply of the antibiotic. That species turned out to be Penicillium chrysogeum, first noticed on a moldy cantaloupe found at a market by lab assistant Mary Hunt. With the help of X-ray and filtration, it could create up to 1,000 times more penicillin than Fleming’s original mold juice.

The timing of penicillin’s discovery and approval for human use coincided with the outbreak of World War II. Penicillin proved to be a miracle worker during the war, and U.S. production increased from 400 million units in the first half of 1942 to 650 billion units a month by 1945. During the First World War, infection had been a major killer, but penicillin dramatically lessened the risk of dying of infection during World War II. For example, bacterial pneumonia’s 18% death rate during WWI dropped to less than 1% during WWII.

More than just mold juice:

  • In 1940, 2,000 liters of mold culture fluid were needed to create enough penicillin to treat just one case of sepsis.

  • In 1942, Anne Miller became the first civilian successfully treated with penicillin, in New Haven, Connecticut. Miller had developed blood poisoning after a miscarriage.

  • Fleming, Florey, and Chain were jointly awarded the Nobel Prize in Physiology or Medicine in 1945 “for the discovery of penicillin and its curative effect in various infectious diseases.” Heatley’s contributions were overlooked, though Oxford University did recognize him with a very rare honorary doctorate of medicine in 1990.

  • In 1999, Time magazine named Fleming to their list of the 100 most important people of the 20th century.

WiseGeek is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
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