How did Alexander Fleming discover penicillin in 1928?

In 1928, Alexander Fleming made his landmark discovery of penicillin through a chance observation at St. Mary’s Hospital. On September 3, he noticed that a culture plate of Staphylococcus aureus bacteria had been contaminated by a fungus.

Upon closer inspection, Fleming observed that the mold—later identified as Penicillium notatum—had inhibited the growth of the surrounding bacteria. Initially referring to the substance as "mould juice," he investigated it further, believing he had discovered a potent enzyme. He eventually determined that the mold produced an antibiotic, which he named penicillin.

While Fleming recognized the clinical potential of the substance as both a topical antiseptic and an injectable treatment, he was unable to stabilize or purify it on his own. The therapeutic development of the drug was later achieved by a team led by Howard Walter Florey and Ernst Boris Chain, with whom Fleming shared the 1945 Nobel Prize for Physiology or Medicine.12