Who was Alexander Fleming?
Alexander Fleming (1881–1955) was the Scottish bacteriologist who discovered penicillin in 1928, when a Penicillium mold drifted onto a staphylococcus culture and killed the bacteria around it — the chance observation that launched the antibiotic revolution and earned him the 1945 Nobel Prize.
Last updated: October 2026
Working at St Mary's Hospital in London, he noticed what others might have thrown away: a clear ring where bacteria could not grow. He isolated the mold's active substance, named it penicillin, and published — then watched the world ignore it for a decade.
Alexander Fleming was born on 6 August 1881 at Lochfield, a farm near Darvel in Ayrshire, Scotland. He was the third of four children of Hugh Fleming, a farmer, and his second wife Grace. The family lived modestly, and the young Fleming attended a small local school before going on to Kilmarnock Academy. At thirteen he followed his older brothers to London, where he attended the Polytechnic in Regent Street, worked for a time as a shipping clerk, and eventually, encouraged by a brother who was a doctor, decided to study medicine. In 1901 he entered St Mary’s Hospital Medical School in Paddington, London, and he would remain connected with St Mary’s for the rest of his working life.
Fleming was a quiet, reserved man, a poor lecturer by his own reputation but a meticulous laboratory worker with a gift for observation. He qualified as a doctor in 1906 and joined the inoculation department at St Mary’s, working under Almroth Wright, a pioneer of vaccine therapy and a formidable, opinionated figure. Wright’s department was devoted to the study of bacteria and the body’s defences against them, and Fleming learned there the techniques of culturing and identifying microbes that would underpin everything he later achieved. He served in the Royal Army Medical Corps during the First World War, working in a laboratory in Boulogne in France, where he saw at first hand how antiseptics failed to save wounded soldiers from infected wounds, and how the body’s own defences mattered more than the chemicals poured into a wound.
After the war he returned to St Mary’s, where he continued to study the body’s natural defences. In 1922 he made his first notable discovery: lysozyme, a natural antibacterial substance found in tears, saliva, mucus and egg white. He discovered it, by his own account, after a drop of mucus from his own nose fell onto a culture plate and cleared the bacteria around it. Lysozyme was too weak to be useful as a medicine, but the discovery showed Fleming’s characteristic way of working: a prepared mind, an untidy bench, and an openness to what an accident could teach. Six years later, a second accident would make him one of the most famous scientists in the world.
The story of the discovery of penicillin has become one of the most retold anecdotes in science, and its essentials are well documented. In September 1928, returning to his laboratory at St Mary’s after a summer holiday, Fleming found that one of the culture plates on which he had been growing Staphylococcus aureus, a bacterium that causes boils, abscesses and wound infections, had become contaminated by a mould. Around the mould, the bacteria had been dissolved, leaving a clear zone. The mould was identified as a species of Penicillium, then called Penicillium notatum and now usually classified as Penicillium rubens. Fleming understood at once that the mould was secreting something that killed bacteria, and he named the substance penicillin.
Fleming published his findings in 1929 in the British Journal of Experimental Pathology, in a paper that reported penicillin’s power against a range of disease-causing bacteria and its apparent harmlessness to white blood cells and to animals. But he could not turn his discovery into a medicine. Penicillin was unstable and difficult to purify in quantity, and Fleming, who was primarily a bacteriologist rather than a chemist, lacked the team and the resources to solve the problem. For nearly a decade penicillin remained a laboratory curiosity, cited by other researchers but used by nobody. Fleming himself moved on to other work, though he kept a culture of the mould and continued to believe in its promise.
The transformation of penicillin from curiosity to cure was the work of a team at Oxford University led by Howard Florey, an Australian pathologist, and Ernst Chain, a German-born biochemist who had fled the Nazis. Beginning in 1938, Florey and Chain, with their colleagues, purified penicillin, demonstrated its power in mice, and in 1941 gave it to a human patient for the first time: Albert Alexander, an Oxford policeman dying of a spreading staphylococcal infection. The patient improved dramatically, then relapsed and died when the small supply of penicillin ran out, a tragedy that underlined both the drug’s power and the urgency of producing it in quantity. Florey and Chain turned to the United States, where the wartime effort to mass-produce penicillin succeeded on an industrial scale, and by the D-Day landings of June 1944 there was enough penicillin to treat the Allied wounded. Fleming, Florey and Chain shared the 1945 Nobel Prize in Physiology or Medicine.
The arrival of penicillin changed medicine more profoundly than almost any other single discovery of the twentieth century. Before antibiotics, a scratch, a sore throat, a childbirth infection or a battlefield wound could kill, and doctors could do little more than keep a patient clean, warm and comfortable while the body fought alone. Pneumonia, meningitis, syphilis, scarlet fever and infected wounds were among the great killers of the age. Penicillin, and the antibiotics that followed it, turned those infections from frequent death sentences into treatable conditions. Surgery became far safer, because post-operative infection could be prevented and treated. Organ transplantation, complex cancer treatment and the care of premature babies all depend, to this day, on antibiotics to hold infection at bay while the body is at its most vulnerable.
The wartime production of penicillin also changed the pharmaceutical industry. The effort to grow the mould in deep fermentation tanks, to strain the penicillin from the broth and to purify it at scale created the modern antibiotic industry almost from nothing, and it drew governments, universities and companies into a new kind of partnership. The success of penicillin set off a hunt for other antibiotics, and the decades after the war produced streptomycin, tetracyclines, chloramphenicol and many more, each expanding the range of infections that medicine could defeat. The age of antibiotics was, in the words of later historians of medicine, the greatest single gain in human life expectancy ever achieved by medical treatment.
Fleming himself became a public figure, celebrated around the world, knighted in 1944, and honoured in his native Scotland and far beyond. He was by all accounts modest about the discovery, and he consistently credited Florey and Chain with turning penicillin into a drug. He continued to work at St Mary’s, where he became professor of bacteriology, and he remained a fellow of the Royal Society. He died of a heart attack on 11 March 1955 and was buried in St Paul’s Cathedral in London, an honour given to very few scientists. His laboratory at St Mary’s is preserved as the Alexander Fleming Laboratory Museum.
Fleming’s Nobel lecture in 1945 contained a warning that has grown more urgent with every passing decade. He imagined a man with a sore throat who bought penicillin, took too little of it, and thereby educated the bacteria in his throat to resist the drug, passing resistant microbes on to his wife. The story was a parable about antibiotic resistance: bacteria that survive exposure to a drug can evolve and spread resistance, and the more carelessly antibiotics are used, the faster resistance spreads. Fleming saw the danger at the very dawn of the antibiotic age, and he urged that penicillin be used carefully and in adequate doses.
That warning has been vindicated. Resistant bacteria, including methicillin-resistant Staphylococcus aureus, known as MRSA, and drug-resistant tuberculosis, now kill large numbers of people each year, and public health authorities around the world describe antimicrobial resistance as one of the great threats to modern medicine. The pipeline of new antibiotics has slowed even as resistance has grown, and the world is now struggling to preserve the miracle Fleming stumbled on in 1928. His discovery is still celebrated as one of the great moments in the history of science, but his warning is the part of his legacy that the twenty-first century most needs to heed.
- NobelPrize.org, “Alexander Fleming — Biographical.”
- Encyclopaedia Britannica, “Alexander Fleming.”
- Brown, Kevin, Penicillin Man: Alexander Fleming and the Antibiotic Revolution (Sutton, 2004).
- Bud, Robert, Penicillin: Triumph and Tragedy (Oxford University Press, 2007).