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

Example Sentences

The discovery began with an unexpected laboratory result, similar to many famous scientific breakthroughs including X-rays, penicillin, Viagra and modern weight loss medications.

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 Fleming Initiative is named after Alexander Fleming, who discovered penicillin in 1928 and started the antibiotic age.

From BBC • Nov. 18, 2025

We learn, for example, that there’s far more to the penicillin story than the casual history of Alexander Fleming’s eureka moment.

From The Wall Street Journal • Oct. 20, 2025

Inside the medicine cabinet, three Extra Strength Tylenol, a box of Imodium, some Midol, and an old bottle of penicillin, which I’m mildly allergic to.

From "Anthem of a Reluctant Prophet" by Joanne Proulx