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penicillin, any of a group of chemically similar substances obtained from molds of the genus Penicillium that were the first antibiotic agents to be used successfully in the treatment of bacterial infections in humans. The antagonistic effect of penicillin on bacteria was first observed by the Scottish biologist Sir Alexander Fleming in 1928. Although he recognized the therapeutic potential of penicillin, it was not until 1941 that a group of biologists working in England, including Sir H. W. Florey and E. B. Chain, purified the substance and established its effectiveness against infectious organisms and its lack of toxicity to humans. Despite the development of hundreds of different antibiotics in recent decades, penicillin remains important in antibiotic therapy.
Small amounts of the antibiotic were first obtained from strains of the mold species P. notatum grown in fermentation bottles. During World War II need for the drug spurred development of better production methods; in the current method highly productive strains of Penicillium are grown in a cornsteep liquor medium in fermentation vats. The main form of penicillin produced by this method is benzylpenicillin, which, like all penicillins, is a derivative of 6-aminopenicillanic acid. Phenoxymethyl penicillin, which can be given orally because it is resistant to degradation by stomach acid, is produced by the species P. chrysogenum.
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