pen·​i·​cil·​lin | \ ˌpe-nə-ˈsi-lən How to pronounce penicillin (audio) \

Definition of penicillin

1 : any of several relatively nontoxic antibiotic acids of the general formula C9H11N2O4SR that are produced by molds (genus Penicillium and especially P. notatum or P. chrysogenum) or synthetically and are used especially against gram-positive cocci also : a mixture of such acids
2 : a salt or ester of a penicillin or a mixture of such salts or esters

Examples of penicillin in a Sentence

Recent Examples on the Web World War II brought radar, penicillin, nuclear power, synthetic rubber, Jeeps and even duct tape. Andy Kessler, WSJ, 27 Mar. 2022 Historian David Oshinsky, who directs the division of medical humanities at NYU Langone Health, said that the sudden acceptance of mRNA reminded him of the trajectory of penicillin. Felicia Schwartz, WSJ, 4 Feb. 2022 When mothers who have syphilis are treated with penicillin while pregnant, babies are often born without a trace of the disease. Caroline Chen, ProPublica, 16 Nov. 2021 The discovery of penicillin by Alexander Fleming in 1928 changed the course of medicine. Popular Mechanics Editors, Popular Mechanics, 11 Jan. 2022 With the introduction of penicillin, cases began to plummet. Talia Herman, ProPublica, 1 Nov. 2021 In honor of a U.S. Department of Agriculture lab in Peoria that helped develop mold-growing methods that led to large-scale production of penicillin, Penicillium rubens NRRL 1951 is being named the state’s official microbe. Jeremy Gorner,, 28 Dec. 2021 Claire is an outlander (a stranger) from the mid-20th century who, after many years living in the 17th century, has mostly adapted to life among folks unfamiliar with penicillin, automobiles or Dr. Seuss. Washington Post, 30 Nov. 2021 Nearly 100 years ago, Alexander Fleming famously discovered penicillin by chance in a London laboratory. Arnab Chakraborty, Fortune, 27 Oct. 2021 See More

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'penicillin.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

First Known Use of penicillin

1929, in the meaning defined at sense 1

History and Etymology for penicillin

New Latin Penicillium

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The first known use of penicillin was in 1929

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Last Updated

17 Apr 2022

Cite this Entry

“Penicillin.” Dictionary, Merriam-Webster, Accessed 19 May. 2022.

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More Definitions for penicillin


pen·​i·​cil·​lin | \ ˌpe-nə-ˈsi-lən How to pronounce penicillin (audio) \

Kids Definition of penicillin

: an antibiotic that is produced by a mold and is used against disease-causing bacteria


pen·​i·​cil·​lin | \ ˌpen-ə-ˈsil-ən How to pronounce penicillin (audio) \

Medical Definition of penicillin

1 : a mixture of relatively nontoxic antibiotic acids produced especially by molds of the genus Penicillium (as P. notatum or P. chrysogenum) and having a powerful bacteriostatic effect against various chiefly gram-positive bacteria (as staphylococci, gonococci, pneumococci, hemolytic streptococci, or some meningococci)
2 : any of numerous often hygroscopic and unstable amido acids (as penicillin G, penicillin O, and penicillin V) that have a structure in which a four-membered lactam ring shares a nitrogen and a carbon atom with a thiazolidine ring to which it is fused and that are components of the penicillin mixture or are produced biosynthetically by the use of different strains of molds or different media or are synthesized chemically
3 : a salt or ester of a penicillin acid or a mixture of such salts or esters

More from Merriam-Webster on penicillin

Nglish: Translation of penicillin for Spanish Speakers Encyclopedia article about penicillin


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