An antibiotic (pl. antibiotics) is an active ingredient used against bacterial infections. Antibiotics function in two different ways: They inhibit the growth of microorganisms and prevent their proliferation (this mode of action is called bacteriostatic), or they kill the bacteria selectively (bactericidal). Antibiotics are not effective against viruses.

Medicine distinguishes between broad-spectrum antibiotics (against a variety of bacteria) and narrow-spectrum antibiotics (against specific pathogens). Researchers continue to look for new active substances with antibiotic properties, because an increasing number of germs are developing antimicrobial resistance.

In 1928, Alexander Fleming discovered the first and now best-known antibiotic – penicillin, which inhibits the cell division of bacteria. It comes from the fungus Penicillium chrysogenum.

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