Medicine 1929

On the Antibacterial Action of Cultures of a Penicillium

Alexander Fleming

A stray mould killed the bacteria around it — and quietly opened the age of antibiotics.

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In depth · the introduction

A speck of mould drifted onto Fleming's dish, killed the bacteria around it, and revealed the world's first antibiotic.

The big idea

Fleming was studying bacteria when he made a famously lucky mistake: he left some culture dishes out, and a stray mould spore drifted in and grew. Looking again, he saw something odd — in a clear ring around the mould, the bacteria had simply dissolved away. The mould was releasing a substance that killed them, and he named it penicillin.

The truly important part wasn't that something could kill bacteria — bleach does that. It was that this substance killed dangerous bacteria without harming living flesh. That selectivity is what turns a chemical into a medicine instead of a poison.

How it came about

In September 1928, back from holiday, Fleming sorted through a pile of old staphylococcus plates. One was contaminated by a blue-green mould — and around it, the bacterial colonies had been wiped out. Rather than throw the “spoiled” dish away, he studied it.

He grew the mould, tested its juice against many bacteria, and found it stopped the ones that cause sore throats, pneumonia, and wound infections, while leaving others — and animal tissue — unharmed. He published in 1929. But he couldn't extract enough pure penicillin to prove it as a cure, and for a decade the finding sat largely unused, until a team at Oxford turned it into a real drug just as the Second World War created desperate need.

Why it mattered

Before antibiotics, a scratch could turn fatal and pneumonia routinely killed. Penicillin and the drugs it inspired have since saved hundreds of millions of lives and made modern surgery and cancer treatment possible. Fleming also gave an early warning we're still grappling with: use these drugs carelessly and bacteria will evolve to resist them.

A way to picture it

Imagine weeding a garden with a spray that kills the weeds but never touches the flowers or the soil. Ordinary disinfectants are like fire — they destroy weeds and flowers alike. Penicillin was the first weed-killer that knew the difference: it jams the particular way bacteria build their cell walls, a machinery our own cells simply don't have.

Where it sits

Fleming's penicillin opened the antibiotic age, but it was one link in a chain: Pasteur and Koch had shown germs cause disease; Ehrlich had dreamed of a “magic bullet” that strikes the microbe and spares the patient; Florey and Chain turned Fleming's mould into medicine. After them came whole families of antibiotics — and, as Fleming foresaw, the growing problem of resistance that today's medicine races to stay ahead of.

The original document

Original source text

Alexander Fleming · Br. J. Exp. Pathol. 10 (1929): 226–236

While working with staphylococcus variants a number of culture-plates were set aside on the laboratory bench and examined from time to time. In the examinations these plates were necessarily exposed to the air and they became contaminated with various micro-organisms.

It was noticed that around a large colony of a contaminating mould the staphylococcus colonies became transparent and were obviously undergoing lysis.

Subcultures of this mould were made and experiments conducted with a view to ascertaining something of the properties of the bacteriolytic substance which had evidently been produced … To avoid the repetition of the rather cumbersome phrase “mould broth filtrate,” the name “penicillin” will be used.

Penicillin, in regard to infections with sensitive microbes, appears to have some advantages over the well-known chemical antiseptics. … It is non-irritant and non-toxic.

It is suggested that it may be an efficient antiseptic for application to, or injection into, areas infected with penicillin-sensitive microbes.