Penicillin
Benzylpenicillin
A mouldy petri dish changed the course of medicine forever.
The accidental discovery that saved 200 million lives.
Key Facts
- 1Discovered by accident from a contaminated Petri dish in 1928
- 2Estimated to have saved over 200 million lives since introduction
- 3Production scaled from laboratory curiosity to industrial manufacturing during World War II
- 4The beta-lactam ring, its key feature, is a strained four-membered ring whose reactivity is amplified by fusion with the thiazolidine ring
- 5Penicillin-destroying enzymes were identified in laboratory bacteria even before the first clinical trials
The Problem
Why This Molecule Was Needed
Before antibiotics, a simple scratch could be a death sentence. In the 1920s, bacterial infections were the leading cause of death worldwide. Sepsis, pneumonia, and childbed fever killed millions every year. Surgeons operated knowing that post-operative infection was almost inevitable. The few treatments available were nearly as toxic to patients as to bacteria.
What medicine needed was a compound that could exploit a fundamental difference between bacterial and human cells. That compound had been quietly growing on a mould for millennia, waiting to be noticed.
The Discovery
How It Happened
Fleming's famous accident
Alexander Fleming returns from holiday to find a Penicillium mould contaminating a Staphylococcus plate. A clear zone of inhibition surrounds the mould, but he cannot purify the active substance.
Florey and Chain crack purification
Howard Florey and Ernst Boris Chain at Oxford stabilise penicillin via freeze-drying and prove its potency: four mice given lethal Streptococcus doses survive after treatment, while all four untreated controls die.
First human trial: Albert Alexander
Policeman Albert Alexander, dying from a Staphylococcus infection after a rose-thorn scratch, receives the first human dose and improves dramatically. The supply runs out after five days; the infection returns and he dies, proving both the drug's power and the urgent need for mass production.
Wartime mass production
The US War Production Board funds deep-tank fermentation, scaling production from 400 million units in early 1943 to 650 billion units per month by 1945.
Nobel Prize and resistance warning
Fleming, Florey, and Chain share the Nobel Prize. Fleming warns in his acceptance speech that misuse will breed resistance, and within two years penicillin-resistant Staphylococcus strains are widespread in hospitals.
The Molecule
Up Close
Tap any region or group card to explore key structural features.
The Takeaway
Ring strain as a chemical weapon
Penicillin's beta-lactam ring is deliberately unstable. That instability is not a flaw; it is the mechanism of action. The strained ring opens to form an irreversible covalent bond with the bacterial transpeptidase enzyme, mimicking the natural substrate the enzyme evolved to process. This principle, using ring strain as a reactive warhead against a specific target, underpins modern covalent drug design.
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