penicillin
Americannoun
noun
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An antibiotic drug obtained from molds of the genus Penicillium and used to treat or prevent various infections caused by gram-positive bacteria such as streptococcus. Penicillin was the first of a class of antibiotics (whose names end in –icillin) that are derived from it and are active against a broader spectrum of bacteria.
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See Note at Alexander Fleming
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Penicillin was first widely used during World War II.
Other Word Forms
Noun Inflected Forms
Etymology
Origin of penicillin
First recorded in 1925–30; penicill(ium) + -in 2
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Explanation
If you have an ear infection, your doctor may give you penicillin. It's a common kind of antibiotic medicine. Penicillin is credited with saving millions of lives. It was discovered in 1928 by a Scottish scientist who returned from a vacation to find a mold growing in a petri dish of infectious bacteria. There were no bacteria growing around the mold, and he named the antibacterial substance produced by the mold penicillin. About 10 years later, a team of different scientists started to figure out how to purify and mass-produce penicillin. The Latin root, penicillus, or "paintbrush," describes the shape of the mold cells that were used to create the original penicillin.
Vocabulary lists containing penicillin
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
See Examples For:
Some important drugs, including penicillin, rely on small ring-shaped molecules that store large amounts of internal tension.
From Science Daily ● May 20, 2026
Alexander Fleming discovered penicillin after mould accidentally contaminated a laboratory dish and killed surrounding bacteria.
From Science Daily ● Mar. 14, 2026
Common problems that could have been treated with an office visit and a few penicillin pills will blossom into heart-valve infections and organ failure.
From MarketWatch ● Jan. 26, 2026
The U.K. has long been a powerhouse in drug discovery, from the 1928 penicillin breakthrough to the landmark Oxford-AstraZeneca Covid-19 vaccine in 2020.
From The Wall Street Journal ● Dec. 1, 2025
The doctor asked a nurse to bring some penicillin.
From "Educated" by Tara Westover
Some bacteria have developed the ability to break down beta-lactam antibiotics like penicillins and cephalosporins, making them ineffective.
From Science Daily ● Feb. 1, 2024
"The fungus growing on the hedgehog was releasing penicillins," explained Prof Holmes.
From BBC ● Jan. 5, 2022
The second were synthetic penicillins, modified in the lab to resist the bugs' enzymes.
From The Guardian ● Jan. 24, 2013
It steadily gained resistance against entire classes of antibiotics: first the penicillins in the 1960s, then the tetracyclines in the 1980s, and Cipro and its chemical cousins, known as fluoroquinolones, in the 1990s.
From Scientific American ● May 4, 2012
By the middle of the century, Fleming's discovery had spawned a huge pharmaceutical industry, churning out synthetic penicillins that would conquer some of mankind's most ancient scourges, including syphilis, gangrene and tuberculosis.
From Time Magazine Archive
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
