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

Testing revealed the most common species was penicillium chrysogenum, famous because it is used to produce the penicillin antibiotic but it can cause allergic reactions or infections in people with compromised immune systems.

From BBC • Mar. 19, 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

“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