On what date did Fleming return to his lab and notice the contaminated culture plate?

Alexander Fleming returned to his laboratory and noticed the contaminated culture plate on 3 September 1928.

At St Mary's Hospital in London, Fleming and his colleague Daniel Merlin Pryce examined culture plates that had been left while Fleming was away. One plate had an open lid and had been contaminated by a blue-green mould. The bacteria failed to grow around the mould, while colonies farther away grew normally.

That observation led Fleming to investigate the antibacterial substance released by the mould, which he named penicillin. The mould was long identified as Penicillium notatum or P. chrysogenum, but later genetic work identified Fleming's strain as Penicillium rubens.

The date is often confused with 28 September 1928, the date Fleming later gave for waking and recognizing the significance of the discovery. The laboratory return and first observation occurred on 3 September. Fleming subsequently resumed his holiday, continued the experiments in October, presented the work in February 1929 and published it in May 1929.

Fleming's initial observation was not yet a usable medicine. Oxford researchers led by Howard Florey later developed methods to cultivate, purify and test penicillin, while industrial production during the Second World War transformed it into a practical antibiotic.