Penicillin

Penicillin is a family of antibiotics, the first natural antibiotic to be discovered and still among the most widely used. In 1928 Alexander Fleming noticed that a Penicillium mold contaminating one of his culture plates stopped staphylococci from growing around it. A team at Oxford led by Howard Florey and Ernst Chain purified the drug and showed around 1940 that it cured infections in animals and people, and mass production followed during the Second World War. The three shared the 1945 Nobel Prize.

Every penicillin contains a four-membered beta-lactam ring. The drug mimics part of the peptidoglycan building block and binds to the transpeptidase enzymes, called penicillin-binding proteins, that cross-link the bacterial cell wall. Without these cross-links the wall weakens and the growing cell bursts, so penicillin kills bacteria rather than just halting them. Human cells have no peptidoglycan, so the drug is selectively toxic. Natural penicillins G and V work best against gram-positive bacteria such as streptococci; semisynthetic versions such as ampicillin and amoxicillin reach more gram-negative species.

Many bacteria now make beta-lactamases, enzymes that break open the ring, so penicillin is often combined with an inhibitor such as clavulanic acid, and resistant strains such as MRSA need other drugs. Penicillin allergy, usually a rash but rarely life-threatening anaphylaxis, is among the most commonly reported drug allergies.