Penicillin was discovered by Alexander Fleming in 1928. It was actually an accident; Fleming had been doing experiments with bacteria, staphylococci, and left his samples alone while he went on vacation. When he returned, one of his samples had been contaminated with a fungus, and he realized, on closer examination, that the fungus had killed the bacteria. Thus, the antibiotic properties of this mold were discovered.
However, he was not the one to apply it to medicinal purposes in the human body; he actually gave up on the idea, and was convinced that the fungus could not do its work in the harsh environment inside the body. When it came to developing the mold for use in medicine, that is attributed to Howard Walter Florey, Ernst Chain and Norman Heatley.
Strictly speaking, penicillin was "discovered" long before Alexander Fleming's experiments. The use of blue mold on bread to treat infection had been a remedy since the middle ages, and the first published reference to the bacteria-killing attributes of fungus of the penicillin variety was much earlier, in 1875. Nonetheless, because Fleming was the first to publicize his results sufficiently and gain the respect of the scientific community, he is attributed with the "discovery" of penicillin to this day.
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Posted 4524 day ago