The first antibiotic came from a mould

In 1928 Alexander Fleming noticed on a plate of staphylococcus culture a zone in which the bacteria had been destroyed around an accidentally growing mould. He isolated a filtrate from the culture and named the active substance penicillin. He published the results in 1929, describing its action against selected bacteria and its relatively low toxicity to the tissues tested. The discovery did not, however, mean that doctors had a finished medicine the next day.

Fleming had difficulty purifying and stabilising penicillin. It was the team of Howard Florey, Ernst Chain and their colleagues at Oxford who developed methods of obtaining it and carried out the experiments showing its effectiveness in treating infections. Wartime demand accelerated large-scale fermentation, the selection of more productive Penicillium strains and industrial production. In 1945 Fleming, Chain and Florey shared the Nobel Prize.

The sentence "the first antibiotic came from a mould" needs a small qualification. Microorganisms were producing substances that inhibit competitors long before humans did, and antibacterial compounds of synthetic origin were in use before penicillin. Penicillin did, however, become the first antibiotic used on such a scale, and a symbol of a new era in treating bacterial infection.

The story does not end with a lucky accident. Fleming himself warned that doses that were too small could favour the survival of resistant bacteria. Today antibiotic resistance is a reminder that a medicine from a fungus is not an inexhaustible resource. Penicillin does not act on viruses, or on all bacteria. The most important lesson from Fleming's plate concerns the combination of careful observation, the work of many teams and responsible use of a discovery. The mould strain itself was later re-identified; the history of the name Penicillium shows that the taxonomy of a drug-producing organism can change even while the chemical mechanism stays the same. The discovery was the start of a long chain of research, not the magical finale of a happy accident.

What the evidence actually shows

Fleming’s 1929 paper described bacterial growth being inhibited around a Penicillium mould and by filtered culture fluid. That observation began the penicillin story, but it was not yet a usable medicine. The material was unstable and difficult to purify; work by Howard Florey, Ernst Chain, Norman Heatley, and colleagues later enabled therapeutic experiments, production, and clinical use in the early 1940s. “First antibiotic” also depends on definition: microbial antagonism had been noticed earlier, and synthetic antibacterial sulfonamides entered medicine before mass-produced penicillin. The central fact nevertheless stands: the molecule that launched the β-lactam antibiotic era was discovered through a mould metabolite. Modern identification of Fleming’s strain is also more nuanced than simply repeating its historical label, P. notatum, because species concepts and names within Penicillium have changed since the original report.