In September 1928, Alexander Fleming, a Scottish bacteriologist working at St Mary's Hospital in London, returned from a two-week holiday to find his laboratory bench in characteristic disarray. Petri dishes of Staphylococcus bacteria cultures he'd been studying before leaving had been left unstacked and unattended—an oversight that would normally mean throwing out contaminated cultures and starting again. But one contaminated dish caught Fleming's attention. A mould had grown on it, and around the mould, something remarkable had occurred: the bacterial colonies had been destroyed. There was a clear zone where bacteria simply would not grow. Fleming's recognition of this observation's significance, and his decision to investigate rather than discard the contaminated dish, began the antibiotic era—arguably the most significant medical advance in human history.
The substance he would isolate from that mould—penicillin—would save hundreds of millions of lives over the following decades. It would transform wartime medicine, making survivable wounds that would previously have killed through infection. It would make routine surgery safe—without antibiotics to prevent post-operative infections, most modern surgery would be impossible. Yet Fleming's role was only the beginning. Turning his laboratory curiosity into a mass-produced medicine would require a decade of work by other scientists, the industrial might of wartime America, and one of the largest scientific collaborations in history.
The Man Behind the Discovery of Penicillin
Alexander Fleming was born on 6 August 1881 in Lochfield, Ayrshire, Scotland, the seventh of eight children in a farming family. His early life was rural and unremarkable—he attended the small village school and later Kilmarnock Academy before moving to London at thirteen to live with his older brother Thomas, a physician.
Fleming worked for four years in a shipping office before inheriting a small legacy that allowed him to pursue medical training. He enrolled at St Mary's Hospital Medical School in London in 1901. His choice of St Mary's was partly accidental—he'd once played water polo against their team and liked the location—but it proved pivotal. St Mary's had a strong research culture and an exceptional bacteriology department.
After qualifying, Fleming could have pursued lucrative general practice but was recruited to the hospital's Inoculation Department by Almroth Wright—a pioneer of vaccine therapy and typhoid immunization. Wright became Fleming's mentor and shaped his research career. Fleming spent his entire professional life at St Mary's, working as a bacteriologist studying infection and immunity.
His personality was famously reserved and taciturn—colleagues described him as quiet, methodical, and somewhat untidy in the laboratory. This untidiness, usually a fault in experimental science, would prove fortuitous.
The Discovery: Chance and the Prepared Mind
Fleming's discovery of penicillin has been mythologised as pure luck—a careless scientist leaving cultures unattended whilst on holiday. The reality is more nuanced and more interesting.
Fleming had been investigating staphylococci—bacteria causing wound infections, boils, and other conditions. In early September 1928, he had been culturing bacterial colonies on agar plates and had left them on his bench whilst going on holiday. Laboratory conditions were not sterile—windows were often open to the London air, and contamination of cultures was common. When he returned in early September, he began examining old cultures before discarding them.
One plate had been contaminated with mould—likely Penicillium notatum, a common airborne fungus. Around the mould colony was a halo where bacterial growth had been inhibited. This observation required significant expertise to recognize as interesting. Contaminated plates were routine; what was unusual was the bacterial inhibition pattern. Many bacteriologists would have discarded it without a second thought.
Fleming's insight—shaped by years studying bacterial inhibition—was to investigate. He cultured the mould, identified it as Penicillium, and demonstrated that it produced a substance that killed or inhibited many pathogenic bacteria. He named this substance penicillin.
His initial 1929 paper describing penicillin is cautious and empirical. He showed that penicillin inhibited Staphylococcus, Streptococcus, and other bacteria whilst being harmless to human white blood cells. He suggested it might be useful for treating infections and as a laboratory tool for isolating certain bacteria.
What Fleming couldn't do was purify and concentrate penicillin sufficiently for medical use. Penicillin was unstable, difficult to extract, and produced in tiny quantities by the mould. After limited attempts at purification, Fleming essentially set the work aside, occasionally using crude penicillin preparations in the laboratory but not pursuing clinical applications.
The Long Road to Medicine
Fleming's discovery might have remained a laboratory curiosity but for two Oxford scientists: Howard Florey and Ernst Boris Chain. In 1938, Florey, Chain, and their team began systematically investigating antibacterial substances, including Fleming's penicillin. By 1940, they had developed purification methods and produced enough penicillin to test in mice infected with deadly Streptococcus.
The results were extraordinary. Untreated mice died within hours. Treated mice survived. This was the proof that penicillin could work as a systemic antibiotic—not just killing bacteria in a dish but saving infected animals.
In February 1941, they treated their first human patient: Albert Alexander, a policeman dying of a severe infection after scratching his face on a rose thorn. The infection was overwhelming—abscesses on his face and scalp, one eye removed, infections spreading through his body. Within 24 hours of receiving penicillin, his condition improved dramatically. Fever fell, appetite returned, infections began clearing. It was miraculous—except they ran out of penicillin before the treatment was complete. Alexander died a month later as the infection returned.
This devastating outcome revealed the challenge: producing enough penicillin for even one patient required all the penicillin the Oxford team could manufacture. Scaling to treat thousands, or millions, seemed impossible with existing methods.
Industrial Production: From Mould to Medicine
The Second World War made penicillin production urgent. Infected wounds killed soldiers whom surgery could otherwise save. The British and American governments invested massively in solving industrial penicillin production.
American pharmaceutical companies pioneered deep-tank fermentation—growing Penicillium in large vats with controlled conditions, aeration, and nutrients. Optimizing growth conditions, finding better Penicillium strains (a crucial strain was isolated from a mouldy cantaloupe in an Illinois market), and developing chemical extraction methods transformed production. By 1943, production had increased enough to treat Allied soldiers. By 1945, penicillin was widely available.
The impact on military medicine was profound. In World War I, bacterial infections killed many of the wounded. In World War II, penicillin made most wound infections treatable. Pneumonia, previously deadly, became survivable. Syphilis, requiring long toxic mercury treatments, could be cured with penicillin in weeks.
Fleming's Recognition and Legacy
Fleming, Florey, and Chain shared the 1945 Nobel Prize in Physiology or Medicine—Fleming for discovery, Florey and Chain for making it medically useful. Fleming was knighted in 1944 and became an international celebrity, though he remained characteristically modest.
He spent his later years warning about antibiotic resistance—predicting that bacterial evolution would adapt to penicillin if it was misused. His warnings, prescient in 1945, are grimly relevant today as antibiotic-resistant infections increasingly challenge modern medicine.
Fleming died of a heart attack in 1955, aged 73. He's buried in St Paul's Cathedral—rare recognition for a scientist, indicating the profound gratitude for his discovery.
The Antibiotic Era and Its Challenges
Penicillin's success launched the antibiotic era. Streptomycin (1943), tetracycline (1948), and dozens of other antibiotics followed, treating previously deadly bacterial infections. Life expectancy increased dramatically—partly from antibiotics preventing and treating infections that had killed throughout human history.
But Fleming's warning about resistance has materialized. Bacteria evolve rapidly, and antibiotic use creates selection pressure favoring resistant strains. Methicillin-resistant Staphylococcus aureus (MRSA), multi-drug-resistant tuberculosis, and carbapenem-resistant bacteria now cause infections that few or no antibiotics can treat.
The pipeline of new antibiotics has slowed—developing antibiotics is less profitable than developing drugs for chronic conditions, so pharmaceutical investment has declined. We face a scenario where common infections might again become untreatable.
Fleming's discovery gave humanity an extraordinary gift—a few generations where bacterial infections were mostly manageable. How long that gift lasts depends on whether we can develop new antibiotics faster than bacteria evolve resistance, and whether we can reduce antibiotic overuse in medicine and agriculture.
Further Exploration
The Alexander Fleming Laboratory Museum at St Mary's Hospital, London, preserves Fleming's laboratory and tells the penicillin story. The Science Museum holds the original culture plate where Fleming observed penicillin's effects.
For reading, "The Mould in Dr Florey's Coat" by Eric Lax covers the full penicillin story, including the Oxford team's crucial work. Fleming's original 1929 paper is surprisingly readable and available online—a masterclass in careful observation and modest claims.
