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Drugs Information Healthline

How do penicillins work?

Penicillins are a group of antibacterial drugs that attack a wide range of bacteria. They were the first drugs of this type that doctors used. The discovery and manufacture of penicillins have changed the face of medicine, as these drugs have saved millions of lives.
Penicillium fungi are the source of penicillin, which people can take orally or via injection.
People across the globe now widely use penicillins to treat infections and diseases.

Fast facts on penicillin

  • Penicillins were the first antibiotic that doctors used.
  • There are several antibiotics in the penicillin class.
  • Experts credit Alexander Fleming with discovering penicillins.
  • Penicillin works by interfering with bacteria cell walls.
  • Less than 1 percent of people are dangerously allergic to penicillin.


Drugs in the penicillin class work by indirectly bursting bacterial cell walls. They do this by acting directly on peptidoglycans, which play an essential structural role in bacterial cells.
Peptidoglycans create a mesh-like structure around the plasma membrane of bacterial cells, which increases the strength of the cell walls and prevents external fluids and particles from entering the cell.
When a bacterium multiplies, small holes open up in its cell walls as the cells divide. Newly-produced peptidoglycans then fill these holes to reconstruct the walls.
Penicillins block the protein struts that link the peptidoglycans together. This prevents the bacterium from closing the holes in its cell walls.
As the water concentration of the surrounding fluid is higher than that inside the bacterium, water rushes through the holes into the cell and the bacterium bursts.


People generally attribute the discovery of penicillins to Alexander Fleming. The story goes that he returned to his laboratory one day in September 1928 to find a Petri dish containing Staphylococcus bacteria with its lid no longer in place.
The dish had become contaminated with a blue-green mold called Penicillium notatum. Fleming noted that there was a clear ring surrounding the mold where the bacteria had been unable to grow.
By discovering this mold and recognizing its use, Fleming set the wheels in motion to create one of the most useful drugs in medical history.
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