September 28, 1928

Alexander Fleming

Alexander Fleming observes a mould inhibiting staphylococci at St Mary's Hospital

The decisive act was not the contamination. It was Fleming's decision to keep the spoiled plate and culture the invader.

September 1928: a clear zone on a ruined plate

In September 1928, the bacteriologist Alexander Fleming inspected a Staphylococcus culture plate at St Mary's Hospital in London that had been accidentally contaminated by a mould. The bacterial colonies adjoining the mould had lysed or failed to grow, leaving a visible clear zone.

A Penicillium spore had landed on a plate where Fleming was studying staphylococcal variants. He kept it.

What Fleming actually did next

Rather than discarding the plate, Fleming retained and cultured the contaminating mould, then demonstrated that its broth filtrate inhibited several disease-causing bacteria. Earlier researchers had reported microbial antagonism involving moulds; Fleming's distinctive contribution was to culture the invader and experimentally characterise its diffusible antibacterial filtrate.

That observation began his investigation of the substance he would name penicillin — it did not produce a purified medicine that day.

The anniversary that cannot be verified

28 September is widely commemorated as the discovery date, but reliable accounts conflict. The American Chemical Society dates Fleming's return and initial plate-sorting to 3 September, and his own 1929 paper supplies no exact discovery day; no contemporary laboratory notebook fixes the observation to a particular date.

Nothing changed in clinical practice immediately: usable therapeutic penicillin required the Oxford team's work more than a decade later.

Sources

Researched 23 Aug 2026 6 sources 3 audit passes Date disputed

How this was checked

Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →

Date disputed. Sources disagree; the research places it at 1928-09.

What the sources leave uncertain

September 28 is widely commemorated as the discovery date, but reliable accounts conflict. The American Chemical Society dates Fleming’s return and initial plate-sorting to September 3, while Fleming’s contemporary 1929 paper supplies no exact discovery day.
Because no contemporary laboratory notebook or dated record securely fixes the observation to September 28, the best-supported date is September 1928 rather than a particular day.
Modern genomic work classifies Fleming’s preserved strain as Penicillium rubens, but Fleming did not identify it by that modern name on the discovery date.