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 The rise of antibiotic-resistant bacteria was predicted by Alexander Fleming, the Scottish bacteriologist who discovered penicillin. Nicola Twilley, The New Yorker, "When a Virus Is the Cure," 14 Dec. 2020 These are small organic compounds found in nature that have therapeutic properties, such as opium produced by the poppy plant or penicillin from the Penicillium fungi. Monique Brouillette, Wired, "A New Field Guide for Earth’s Wild Microbes," 18 Dec. 2020 From August to October, a dose of penicillin more than doubled in price, while aspirin cost rose by a third, according to a report from the Korea International Trade Association. Andrew Jeong, WSJ, "North Korea’s Economy Hit Harder Than It Has Been in Decades," 18 Dec. 2020 As early as 1947, penicillin-resistant staphylococcus bacteria were found in hospitals in England, but few heeded Fleming’s warning. Nicola Twilley, The New Yorker, "When a Virus Is the Cure," 14 Dec. 2020 Even when penicillin became widely used to treat syphilis in 1947, it was not offered to the test subjects. NBC News, "Racial disparities create obstacles for Covid-19 vaccine rollout," 3 Dec. 2020 What’s the value of the most important medicine since the invention of penicillin? James Mackintosh, WSJ, "There Is No Vaccine Against These Investing Mistakes," 17 Nov. 2020 Few drugs are penicillin-size successes; most drugs have more moderate effects. Adam Rogers, Wired, "The Strange and Twisted Tale of Hydroxychloroquine," 11 Nov. 2020 But for the 2,500 such men returning from World War II, modern wheelchairs offered mobility, and the new wonder drug penicillin cured dangerous infections. Annette Sisco,, "How paralyzed veterans inspired a movement for disability rights, starting with basketball," 8 Nov. 2020

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.

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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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Time Traveler for penicillin

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The first known use of penicillin was in 1929

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Last Updated

23 Jan 2021

Cite this Entry

“Penicillin.” Dictionary, Merriam-Webster, Accessed 25 Feb. 2021.

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More Definitions for penicillin



English Language Learners Definition of penicillin

: a medicine that is used to kill harmful bacteria


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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