penicillin

American  

[pen-uh-sil-in] / ˌpɛn əˈsɪl ɪn /

noun

Pharmacology.

• penicillins
  plural

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.

---

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

---

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

---

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.

---

Discover More

Penicillin was first widely used during World War II.

Other Word Forms

Noun Inflected Forms

• penicillins
  plural

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 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.

Example Sentences

See Examples For:

penicillin penicillins

One of those compounds was probenecid, a medication originally introduced in the 1940s to help conserve scarce supplies of penicillin by reducing how much of the antibiotic was excreted in urine.

From Science Daily ● Jun. 17, 2026

Some important drugs, including penicillin, rely on small ring-shaped molecules that store large amounts of internal tension.

From Science Daily ● May 20, 2026

Lifeline follows the story of Sir Alexander Fleming's discovery of penicillin at St Mary's Hospital in Paddington in 1928, weaving his historical journey with a contemporary narrative centred on a junior doctor in Edinburgh.

From BBC ● Apr. 5, 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

“I can give you penicillin for the strep, but there’s not much I can do for the mono. You’ll have to wait it out. Still, once we’ve cleared out the strep, you should feel better.”

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

Sulfonamides ceased to be effective in treating it in the 1940s; penicillins and tetracyclines lost effectiveness in the 1970s and ’80s; and fluoroquinolones were taken off the treatment table in 2007.

From Scientific American ● Jan. 8, 2013

Last week the British were saluting the arrival of a new penicillin prince, Penbritin,* which promises to carry the fight against groups of microbes that have defied all previous penicillins.

From Time Magazine Archive