Alexander Fleming was a Scottish bacteriologist who discovered penicillin in 1928, launching the age of antibiotics and changing medicine forever. Born in 1881, he spent most of his career at St. Mary’s Hospital in London, where a contaminated petri dish led to one of the most important accidental discoveries in scientific history. He shared the 1945 Nobel Prize in Physiology or Medicine for this work and died in 1955, buried in St. Paul’s Cathedral.
Early Life and Career at St. Mary’s
Fleming grew up in rural Scotland before moving to London, where he studied and eventually worked at St. Mary’s Hospital Medical School. He was known among colleagues for being brilliant but disorganized, a trait that would, ironically, play a direct role in his two major discoveries. His early career was shaped by the devastation of World War I, where he witnessed soldiers dying not from their battlefield wounds but from the infections that followed. During the Great War, between 12% and 15% of wounded soldiers treated in front-line hospitals died of bacterial infections, most notably gas gangrene. The antiseptics available at the time were crude and often did more harm than good, damaging healthy tissue along with bacteria. This experience left Fleming determined to find something better.
Lysozyme: The First Discovery
In 1922, Fleming stumbled onto his first significant find. While suffering from a cold, he transferred some of his nasal mucus onto a petri dish. True to form, he left the dish sitting on his cluttered desk for two weeks. When he finally looked at it, bacteria had grown across most of the plate, but the area where the mucus had landed remained completely clear. Something in the mucus was stopping bacteria from growing.
Fleming identified the substance responsible and named it lysozyme, a natural enzyme the body produces as a first line of defense against germs. He found it in tears, saliva, skin, hair, and fingernails, and discovered that egg whites contained especially large amounts. The catch was that lysozyme only worked against a small number of harmless bacteria. It couldn’t touch the dangerous ones that caused serious infections. Still, it proved an important principle: the body itself produced substances capable of killing microbes. And it primed Fleming to recognize something similar when it appeared again six years later.
The Discovery of Penicillin
The story of penicillin’s discovery in September 1928 has become one of science’s most famous tales. Fleming had returned from vacation to his laboratory at St. Mary’s and was sorting through petri dishes he’d left out before leaving. While talking to a colleague, he noticed something unusual on one of the plates. A mold had contaminated a dish where he’d been growing staphylococcus bacteria, and in the zone immediately surrounding the mold, the bacteria had been killed off.
Rather than tossing the contaminated dish, Fleming investigated. He isolated the mold and identified it as belonging to the Penicillium genus (it was initially classified as P. rubrum but later corrected to P. notatum). He extracted a substance from the mold and named its active agent penicillin. In laboratory tests, it proved remarkably effective at stopping the growth of several types of dangerous bacteria.
Fleming published his findings, but there was a problem he couldn’t solve. Penicillin was extremely difficult to produce in usable quantities, and it was unstable. Fleming lacked the resources and the chemistry expertise to turn a promising laboratory observation into an actual medicine. His paper attracted little attention at the time, and the discovery sat largely dormant for over a decade.
From Lab Curiosity to Life-Saving Drug
The transformation of penicillin from a mold on a petri dish into the world’s first mass-produced antibiotic was the work of two other scientists: Howard Florey, an Australian pathologist, and Ernst Chain, a German-born biochemist, both working at the University of Oxford. In 1938, Chain came across Fleming’s earlier paper on penicillin while researching lysozyme at Florey’s suggestion. The two began the painstaking process of extracting and purifying the drug, initially growing the mold in porcelain vessels.
Their first human patient, treated in January 1941, was an Oxford policeman suffering from severe infections caused by a scratch from a rose thorn. He improved dramatically on penicillin but relapsed when the tiny supply ran out. Over the next three months, five more patients were treated with impressive results, and Florey published the findings in The Lancet in August 1941. Because wartime England couldn’t produce enough penicillin for full clinical trials, Florey and biochemist Norman Heatley traveled to the United States in June 1941 to scale up production with American help.
The timing was critical. By the latter stages of World War II, penicillin was being manufactured on an industrial scale, and the impact was staggering. The mortality rate from wound infections dropped from the 12% to 15% seen in World War I down to roughly 3% in World War II. The total number of lives saved is, by most accounts, incalculable.
The Nobel Prize and a Prophetic Warning
In 1945, Fleming, Florey, and Chain shared the Nobel Prize in Physiology or Medicine “for the discovery of penicillin and its curative effect in various infectious diseases.” Fleming was celebrated worldwide, though he was careful to credit Florey and Chain for making the drug usable.
During his Nobel Lecture in December 1945, Fleming issued a warning that sounds remarkably modern. “The time may come when penicillin can be bought by anyone in the shops,” he said. “Then there is the danger that the ignorant man may easily underdose himself and by exposing his microbes to non-lethal quantities of the drug make them resistant.” Nearly 80 years later, antibiotic resistance is one of the most serious threats to global health, driven by exactly the kind of misuse Fleming predicted: incomplete courses of treatment, unnecessary prescriptions, and widespread use of antibiotics in agriculture.
Fleming’s Place in Medical History
Fleming died on March 11, 1955, and was buried in St. Paul’s Cathedral in London, an honor reserved for the nation’s most distinguished figures. His legacy is complicated in an interesting way. He made the initial observation that launched the antibiotic era, but he could not have turned it into a medicine alone. Florey and Chain, along with contributors like Norman Heatley and Edward Abraham, did the grinding biochemical work of extraction, purification, and clinical testing. Fleming himself acknowledged this.
What set Fleming apart was a quality that sounds simple but is rare in science: he paid attention to anomalies. Twice in his career, a messy lab bench produced something unexpected, and twice he stopped to ask why. Most researchers would have discarded a contaminated petri dish. Fleming’s willingness to look closer at what everyone else would have thrown away gave the world a class of drugs that has saved an estimated 200 million lives and counting.