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Antibiotic, any of the substances, usually produced by microorganisms, that kill or prevent the growth of other microorganisms, especially bacteria and fungi. Louis Pasteur noted the effect in the 19th century, and in 1928 Alexander Fleming showed that the mold Penicillium notatum produced penicillin, a substance able to destroy certain bacteria. Other early discoveries of antibiotics include the isolation of streptomycin (by Selman Waksman), gramicidin (by René Dubos), and the cephalosporins. Semisynthetic antibiotics, in which the basic molecule is chemically modified, have increased the range of naturally occurring substances.

See also: Penicillin; Bacteria.

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