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Science · Bacteriology & Medicine
Alexander Fleming
Discovered penicillin in 1928 — saved an estimated 200 million lives
In September 1928, a Scottish bacteriologist returned from his summer holiday to a cluttered laboratory at St Mary's Hospital in London, and noticed something unusual on a petri dish he had left behind. A mould had contaminated one of his cultures of Staphylococcus bacteria — and around the mould, the bacteria were dead. Alexander Fleming did not immediately grasp that he had just made the most important medical discovery of the twentieth century. But he was curious enough to look more carefully, and careful enough not to throw the dish away. That decision would eventually save hundreds of millions of lives.
Alexander Fleming was born on August 6, 1881, at Lochfield Farm in Ayrshire, Scotland, the seventh of eight children. His father died when he was seven. At thirteen he moved to London to live with an elder brother, worked briefly in a shipping office, and then, having received a small inheritance and a nudge from his brother (also a physician), enrolled at St Mary's Hospital Medical School in 1901. He qualified as a doctor in 1906 and joined the Inoculation Department under Almroth Wright, one of the leading bacteriologists of the age. He would remain at St Mary's for virtually his entire career — nearly fifty years.
Fleming's early research was shaped by his experience as a medical officer in World War One, during which he witnessed vast numbers of soldiers dying not from their wounds but from bacterial infections — streptococcal and staphylococcal infections that modern antibiotics would now cure in days. He became preoccupied with the problem of bacterial killing. In 1922, he discovered lysozyme, an enzyme present in tears, saliva, and mucus that has mild antibacterial properties. It was an important finding — the first evidence that the body produced natural antibacterial substances — but lysozyme was not potent enough to treat serious infections.
The contaminated petri dish in 1928 was something different. Fleming observed that the mould — later identified as Penicillium notatum — appeared to produce a substance that killed bacteria in a wide radius around it. He isolated the mould, grew it in cultures, and tested the broth it produced against a range of bacterial species. The results were remarkable: the substance he called "penicillin" was powerfully effective against Staphylococcus, Streptococcus, and several other dangerous pathogens, while appearing harmless to human white blood cells. He published his findings in the British Journal of Experimental Pathology in 1929.
Fleming lacked the chemistry expertise to purify and concentrate penicillin, and his early attempts to produce it in therapeutic quantities failed. For more than a decade, his discovery remained a laboratory curiosity — known but unused. The critical step came in the early 1940s, when Howard Florey and Ernst Boris Chain at Oxford University took Fleming's published results seriously, developed methods to purify and concentrate the drug, and conducted the first clinical trials. Their results were extraordinary: patients dying from infections that had been untreatable were recovering within days. With wartime urgency, the United States and Britain mobilized mass production. By the D-Day landings in June 1944, penicillin was available in sufficient quantities to treat Allied casualties.
In 1945, Fleming, Florey, and Chain shared the Nobel Prize in Physiology or Medicine — a rare instance of a Nobel being awarded so rapidly, and so justified by the scale of human benefit involved. Fleming became a global celebrity, feted by heads of state, given honorary degrees, and mobbed by admirers wherever he traveled. He used his fame to issue a warning: the misuse of penicillin would breed resistant bacteria. He was right. Antibiotic resistance is now one of the gravest threats facing global health — a problem that Fleming predicted in his Nobel Prize acceptance speech.
Fleming died of a heart attack on March 11, 1955, at his home in London, and was buried in St Paul's Cathedral. The petri dish that launched his discovery — preserved and displayed — has become one of the most iconic objects in the history of science. His name appears on hospitals, research institutes, and medical schools across the world. The number of people alive today because of penicillin and its descendants is incalculable.
| Born | August 6, 1881, Lochfield, Ayrshire, Scotland |
| Died | March 11, 1955 (aged 73), London, England |
| Field | Bacteriology, immunology, pharmacology |
| Key Discovery | Penicillin, 1928 |
| Earlier Discovery | Lysozyme, 1922 |
| Nobel Prize | Physiology or Medicine, 1945 (with Florey and Chain) |
| Institution | St Mary's Hospital Medical School, London |
| Buried | St Paul's Cathedral, London |
| Scientist | Field | Core Contribution | Era |
|---|---|---|---|
| Alexander Fleming | Bacteriology | Discovery of penicillin | 20th century |
| Louis Pasteur | Microbiology | Germ theory, pasteurization, vaccines | 19th century |
| Edward Jenner | Medicine | First vaccine — smallpox | 18th century |
| Robert Koch | Bacteriology | Identified tuberculosis & cholera bacteria | 19th century |
Before penicillin, a scratch from a rose thorn could kill. Pneumonia was a death sentence. Strep throat killed children. Soldiers died of infected wounds more often than of the wounds themselves. Fleming's accidental observation — and his disciplined curiosity to follow it — unlocked a class of medicines that transformed what it means to be ill. Antibiotics have saved more lives than any other class of pharmaceutical in history.
His warning about resistance is equally important. Fleming foresaw, in 1945, exactly the crisis now facing global health: bacteria evolving to defeat the drugs that once killed them. Every call for responsible antibiotic use echoes the Nobel lecture of the man who started it all. He did not just give humanity a weapon; he told us how to keep it sharp.