In short: Alexander Fleming discovered penicillin in 1928 when a mould called Penicillium notatum accidentally landed on one of his bacterial culture plates. The mould killed the surrounding bacteria, leading to the world's first antibiotic — a medicine that has saved millions of lives.
How was penicillin discovered?
In the summer of 1928, a Scottish bacteriologist named Alexander Fleming was growing bacteria in his lab at St. Mary's Hospital in London. Before he went on a two-week vacation, Fleming left a petri dish of Staphylococcus bacteria sitting on a lab bench instead of storing it properly. He did not expect it to matter — he thought he would just clean it up when he returned.
But something unexpected happened. While Fleming was away, a mould spore floated into his lab — perhaps drifting through a window or down from another lab in the building. The spore landed on his petri dish and began to grow. When Fleming came back, he noticed something remarkable: around the mould, the bacteria had all died. There was a clear ring of empty space where the bacteria used to be.
"That's funny," Fleming said — or so the story goes — and he decided to investigate.
What was the mould?
Fleming isolated the mould and studied it carefully. He identified it as Penicillium notatum, a fungus in the same family as the blue-green mould that sometimes grows on bread or fruit. The name "Penicillium" comes from the Latin word penicillus, which means paintbrush — because the fungus looks like tiny paintbrushes under a microscope.
But this mould was special. It made a chemical that killed many of the dangerous bacteria that infect humans. Fleming called this chemical penicillin. He published his discovery in 1929, but at first, many scientists did not realize how important it was.
Safety first: Never pick or eat wild mushrooms or mould, even if they look interesting. Many moulds and wild fungi are dangerous or toxic. The safe way to grow and see fungi is with a grow kit or in a lab with proper training. Look at wild fungi with your eyes only — never pick or touch them without an expert adult.
How does penicillin work?
Penicillin stops bacteria from building their protective walls. All bacteria are wrapped in a tough cell wall, like a skin. Penicillin breaks apart the molecules that hold the bacteria's wall together, so the cell wall falls apart. The bacterium fills with water and bursts. This works on many kinds of bacteria that cause infections — like the Staphylococcus bacteria Fleming was studying.
Because Fleming's bacteria was Staphylococcus aureus, a germ that causes painful infections, his discovery meant there was finally a weapon against it.
Why did it take so long to use penicillin as medicine?
Fleming had discovered penicillin, but turning it into a medicine was much harder. The chemical was fragile and difficult to purify. Fleming tried for years, but could not figure out how to make enough of it or keep it stable. By the late 1930s, other scientists took up the challenge — especially Ernst Boris Chain and Howard Florey at Oxford University. Together, they managed to purify penicillin and test it. By 1941, the first injectable penicillin was ready for doctors to use. During World War II, penicillin prevented countless deaths by stopping infections in wounded soldiers that would have been deadly just years before.
What did Fleming win for this discovery?
In 1945, Alexander Fleming, Ernst Chain, and Howard Florey shared the Nobel Prize in Physiology or Medicine — one of the world's highest honours for science — for discovering and developing penicillin. Fleming's accidental discovery had changed the course of medicine forever.
The short version
In 1928, Alexander Fleming left a petri dish on his lab bench before going on vacation. A mould called Penicillium notatum accidentally landed on it and killed all the bacteria around it. Fleming realised the mould was making a powerful chemical he called penicillin. It took many years and help from other scientists to turn it into a medicine, but penicillin became the world's first antibiotic — a drug that fights dangerous bacterial infections. It has saved millions of lives and changed medicine forever.
Curious questions
What is penicillin?
Penicillin is a chemical made by a mould called Penicillium notatum. It kills many kinds of dangerous bacteria that cause infections. It is an antibiotic — a medicine that fights bacterial diseases.
Was the penicillin discovery really an accident?
Yes! Alexander Fleming left a petri dish on his lab bench by mistake before a vacation. If he had cleaned it up properly, or if a mould spore had not drifted in, penicillin might never have been discovered.
Can I grow Penicillium notatum at home?
You can see moulds grow naturally in the right conditions — like on fruit left too long — but you should never handle them without an adult. Moulds can be dangerous. The safe way to learn about fungi is with a proper grow kit designed for kids.
Does penicillin work on all bacteria?
No. Penicillin works best on a group called gram-positive bacteria, which includes germs like Staphylococcus. Some bacteria have defences against penicillin, which is why scientists keep developing new antibiotics.
Why did it take so long to turn penicillin into a medicine?
The penicillin chemical was very fragile and hard to purify. Fleming could not figure out how to make enough or keep it stable. It took other scientists like Ernst Chain and Howard Florey to solve the problem in the late 1930s and early 1940s.
Sources & further reading
- Science History Institute — Alexander Fleming biography
- University of Wisconsin Botany Dept — Penicillium notatum and penicillin history
- National Institutes of Health (PMC) — The Discovery of Penicillin
A note for growers: only grow and eat mushrooms from our kits or from species you know are safe, and always check with an adult. Never eat a wild mushroom unless an expert adult says it’s safe.
