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September 1928General source · 2 sourcesWell documented

Alexander Fleming Discovers Penicillin in a Contaminated Petri Dish

A stray mold spore kills a bacterial culture Fleming left uncovered, and the observation becomes the first antibiotic once Florey and Chain turn it into a drug

On the timeline · around September 1928 · Modern Pandemics and Modern MedicineThe Birth of Vaccination and EpidemiologyModern Pandemics and Modern MedicineAlexander Fleming Discovers Penicillin in a Contaminated Petri Dish1890191019201930194019501960

Quick facts

Discoverer
Alexander Fleming
Location
St Mary's Hospital, London
Date
September 1928, observed; published 1929
Mold
Penicillium notatum
Made into a usable drug by
Howard Florey and Ernst Chain, Oxford, 1940

What happened

In September 1928, Alexander Fleming, a bacteriologist at St Mary's Hospital in London, returned from a two-week vacation to find that a petri dish of Staphylococcus bacteria he had left uncovered on his bench had been contaminated by a mold, likely drifting up from another lab in the building or in through a window. The temperature conditions during his absence had allowed both the bacteria and the mold to grow side by side, and Fleming noticed a clear zone around the mold where the surrounding bacterial colonies had been killed or stopped from growing. He identified the mold as Penicillium notatum and named the antibacterial substance it produced penicillin. Fleming published the finding in 1929, but it drew little scientific attention at the time and he did not pursue it as a treatment. The breakthrough into a usable drug came over a decade later, when Oxford researchers Howard Florey and Ernst Chain isolated and purified enough penicillin to demonstrate its therapeutic effect in 1940.

Why it matters

Penicillin became the first true antibiotic, a drug that kills bacteria without also poisoning the patient, and its mass production during the Second World War transformed the treatment of infections that had routinely killed people from wounds, pneumonia, and childbirth complications. The gap between Fleming's 1928 observation and Florey and Chain's 1940 purification also illustrates how a discovery and a usable medicine are not the same thing; both steps were necessary.

How we know

Fleming's 1929 paper in the British Journal of Experimental Pathology documents the original observation; the Science History Institute's biographical account, drawn from historical records of the discovery, traces the subsequent development through Florey and Chain's 1940 work.

Sources

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