penicillin

American  

[pen-uh-sil-in] / ˌpɛn əˈsɪl ɪn /

noun

Pharmacology.

1. any of several antibiotics of low toxicity, produced naturally by molds of the genus Penicillium and also semisynthetically, having a bactericidal action on many susceptible Gram-positive or Gram-negative cocci and bacilli, some also being effective against certain spirochetes.

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penicillin British  

/ ˌpɛnɪˈsɪlɪn /

noun

1. any of a group of antibiotics with powerful bactericidal action, used to treat many types of infections, including pneumonia, gonorrhoea, and infections caused by streptococci and staphylococci: originally obtained from the fungus Penicillium, esp P. notatum. Formula: R-C 9 H 11 N 2 O 4 S where R is one of several side chains

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penicillin Scientific  

/ pĕn′ĭ-sĭl′ĭn /

1. 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.

2. See Note at Alexander Fleming

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penicillin Cultural  

1. An antibiotic that is used to treat infections caused by some kinds of bacteria. Penicillin, which is derived from a common kind of mold that grows on bread and fruit, was the first antibiotic discovered and put into widespread use.

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Discover More

Penicillin was first widely used during World War II.

Etymology

Origin of penicillin

First recorded in 1925–30; penicill(ium) + -in ²

Compare meaning

How does penicillin compare to similar and commonly confused words? Explore the most common comparisons:

• ampicillin vs. penicillin
• penicillin vs. amoxicillin

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 ten 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.

Example Sentences

Alexander Fleming discovered penicillin after mould accidentally contaminated a laboratory dish and killed surrounding bacteria.

From Science Daily • Mar. 14, 2026

It’s around 3 p.m. on a Thursday, and Joni Gonzalez, a nurse, has just learned to make the perfect batch of penicillin.

From The Wall Street Journal • Mar. 6, 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

In one way, it’s funny to hear C-suite executives describing this fried appetizer iteration in the kinds of awestruck tones usually reserved for, like, the discovery of penicillin.

From Slate • Aug. 11, 2025

“All right, so, B. cepacia is a hardy bacterium. It’s so adaptive that it actually feeds on penicillin instead of being attacked by it. So our first line of defense is...”

From "Five Feet Apart" by Rachael Lippincott