How Did Alexander Fleming Discover Penicillin? A Messy Lab, a Mould and a Prepared Mind

Alexander Fleming discovered penicillin in 1928 when he came back from a holiday to his laboratory at St Mary's Hospital in London and noticed that a mould had contaminated one of his culture plates of bacteria, and that the bacteria around the mould had been killed. He named the bacteria-killing substance penicillin. The discovery was partly luck, but it took a scientist who was already looking for ways to fight infection to recognise what he was seeing, and it took another team, years later, to turn it into a medicine.

A farmer's son from Scotland

Fleming was born on 6 August 1881 on a farm near Darvel in Ayrshire, Scotland. He moved to London as a teenager, worked for a time in a shipping office, and then, helped by a small inheritance, studied medicine at St Mary's Hospital Medical School. He stayed at St Mary's for almost his entire career, working in its research department.

What the First World War taught him

During the First World War Fleming worked in military hospitals in France and saw huge numbers of soldiers die from infected wounds. He noticed something troubling: the antiseptics commonly used to treat deep wounds often made things worse, damaging the body's own defences while failing to reach the bacteria deep inside. After the war he kept searching for substances that would kill bacteria without harming the patient.

His first real discovery came in 1921–22: lysozyme, an enzyme with antibacterial properties found in body fluids such as nasal mucus and tears. Lysozyme turned out to be too weak against the most dangerous bacteria, but it trained Fleming to look for natural antibacterial substances.

September 1928: the famous plate

Fleming had a reputation for an untidy laboratory. Before going on holiday in the summer of 1928, he left culture plates of Staphylococcus bacteria stacked on a bench. When he returned, he sorted through them and noticed one that had been contaminated by a mould. Around the mould was a clear zone where the bacteria had been destroyed.

Many scientists would have thrown the plate away as spoiled. Fleming, after years of hunting for natural antibacterial agents, recognised its importance. He grew the mould, a species of Penicillium, and found that the "mould juice" could kill many kinds of harmful bacteria. In 1929 he named the active substance penicillin and published his findings.

The problem nobody could solve (yet)

Penicillin had a serious practical problem: it was very difficult to purify and produce in useful amounts, and it was unstable. Fleming was not a chemist, and his attempts to isolate it made slow progress. As a result, penicillin received little attention through the 1930s.

Florey and Chain finish the job

The breakthrough came at the University of Oxford, where a team led by Howard Florey and Ernst Chain took up penicillin around 1939–1940. They found ways to purify it, tested it on infected mice with dramatic results, and then treated the first human patients.

With the Second World War raging, the need was urgent. Production was scaled up, much of it in the United States, and penicillin was mass-produced in time to treat Allied soldiers. Infections that had been death sentences became treatable.

Fame and a Nobel Prize

Fleming became a celebrity, often presented as the lone genius behind the "miracle drug", a simplification that he himself did not entirely correct. He was knighted in 1944, and in 1945 he shared the Nobel Prize in Physiology or Medicine with Florey and Chain. The shared prize is the fairer version of the story: one man noticed, a team made it work.

An early warning about resistance

Fleming also saw a danger very early. He warned that misusing penicillin, for example by taking too little or stopping too soon, could allow bacteria to become resistant to it. Today antibiotic resistance is one of the major challenges in medicine, which makes his warning seem remarkably far-sighted.

He died in London on 11 March 1955.

Learn with Fleming: English phrases about discovery

Phrase	Meaning	Example
a happy accident	a mistake that leads to something good	The new recipe was a happy accident.
to stumble across	to find by chance	I stumbled across a great grammar video.
to take something up	to start working on something	The Oxford team took up the research.
to scale up	to increase in size or amount	They scaled up production during the war.
a word of warning	advice about a possible danger	A word of warning: don't skip the basics.

Try this: Write three English sentences about a "happy accident" in your own learning, a mistake that taught you something useful. Use "stumble across" and "a word of warning".