Fleming penicillin discovery 1928

Fleming penicillin discovery 1928 remains one of the most retold stories in science — and like most retold stories, the version most of us grew up with is cleaner, faster, and more satisfying than what actually happened.

The version I learned went roughly like this: Fleming went on vacation, forgot to clean up his lab, came back to find mold growing in a petri dish, noticed the mold was killing bacteria, and — eureka — the antibiotic age began.

I believed this story for a long time. It has the right shape: a lucky accident, a curious mind, a world-changing discovery. It feels complete. And the broad outlines are real — the vacation, the contaminated dish, the observation, all of it happened. What the story leaves out is that none of it produced penicillin. What it produced was a curious note in a laboratory notebook and a paper the scientific community largely ignored for eleven years, while people kept dying of infections it could have treated.

The drug that actually saved your grandparents’ generation was created not by Fleming, but by three other scientists at Oxford who read his forgotten paper in 1939 and decided to take it seriously. And even then, the process involved bedpans, milk churns, a rotting cantaloupe from an Illinois supermarket, and a production process one manufacturer described as: “The mold is as temperamental as an opera singer, the yields are low, the isolation is difficult, the extraction is murder, the purification invites disaster, and the assay is unsatisfactory.”

When I first came across the full story, my reaction wasn’t disappointment that the legend was wrong. It was something closer to recognition — the familiar feeling that the interesting part of almost any breakthrough isn’t the moment of insight. It’s the long, unglamorous, frequently humiliating work that happens between the observation and the application. That middle part is where most good ideas die. And in the case of penicillin, it almost did.

Fleming penicillin discovery 1928 mold petri dish

Table of Contents

The Man and the Mess

Alexander Fleming was born in 1881 in rural Scotland, the son of a farmer. He spent four years working in a shipping office before an inheritance from his uncle allowed him to fund medical school. He graduated from St. Mary’s Hospital Medical School in London in 1906 with distinction and spent essentially his entire career there.

By all accounts, Fleming was a warm, quiet, genuinely modest man who had an extraordinary gift for patient observation and a well-documented indifference to laboratory tidiness. I want to linger on that second quality for a moment, because it’s usually mentioned as a charming quirk and then moved past — but it’s actually the hinge on which the entire story turns. Fleming was not simply untidy. He had a specific habit of leaving experiments sitting around and going back to look at them again, days or weeks later, before finally clearing them away. One colleague described him as someone who “always examined his experiments before discarding them, even those left for weeks.”

In most laboratories, this would be considered poor practice. In Fleming’s case, it was the precise characteristic that made the discovery possible. If he’d run a cleaner operation, the petri dish would have been washed before the mold had a chance to do anything interesting.

There’s something I find both funny and slightly pointed about this. We spend a lot of time in any technical field talking about rigor, discipline, and systematic methodology. Fleming’s most important discovery was enabled by his messiness. I’m not suggesting messiness is generally a virtue — it usually isn’t — but I do think there’s something worth sitting with in the idea that the same qualities that make a person unconventional in some ways can make them uniquely positioned to notice things that more methodical people don’t see.

In August 1928, Fleming left his laboratory at St. Mary’s for a two-week vacation with his family. Before leaving, he’d set up several petri dishes containing colonies of Staphylococcus aureus — the common bacteria responsible for boils, skin infections, and in serious cases, potentially fatal blood poisoning. He left the dishes on his bench rather than placing them in an incubator, pushed to one corner to make space for a colleague. He did not cover them properly. Then he left.

What Happened While Nobody Was Watching

The Fleming penicillin discovery of 1928 hinged on a sequence of events so precisely timed that recreating it took another scientist nearly forty years of trying.

The story of what happened next involves a piece of luck so precisely calibrated that when a scientist later tried to recreate it, he spent years trying before he succeeded — and he only succeeded because he happened to attempt it during an unusual cold snap in 1966 that accidentally reproduced the weather conditions of August 1928.

While Fleming was away, two things happened in his London lab in a very specific sequence. First, a mold spore — later identified as Penicillium notatum, a relatively rare strain — somehow found its way onto one of the uncovered petri dishes. It came either through an open window, or more likely drifting up a stairwell from a laboratory on the floor below where various molds were being cultured for unrelated research.

Second, the weather in London that August was unusual. A heat wave in mid-August was followed by an abrupt cold snap — nine days of cooler temperatures starting around August 28. This mattered because Penicillium mold grows well in cool conditions, while Staphylococcus bacteria prefer warmth. The specific temperature sequence that summer allowed the mold to establish itself and begin producing its antibacterial compound before the bacteria had fully colonized the plate. A window of perhaps a week or two. After which the bacteria would have overgrown everything and the result would have been invisible.

I’ve thought about this a lot, and I keep coming back to the sheer numerical improbability of it. The right mold spore, from the right strain, drifting from the right laboratory downstairs, landing on the right uncovered dish, in the right temperature conditions that only existed for nine specific days that particular summer. If Fleming had taken his holiday two weeks earlier, or the cold snap had arrived two weeks later, or the dish had been covered, or the mold spore had been a different species — the discovery doesn’t happen. Not then. Not like that. I’m not a fatalistic person, but the specificity of what had to align genuinely makes me pause. The universe was not obligated to arrange things this way. It just happened to.

The Moment Fleming Said “That’s Funny”

Fleming returned to his laboratory on September 3, 1928, and began going through his old culture plates before discarding them — as was his habit. When he picked up one particular plate, something was wrong with it. A large patch of greenish mold had established itself on one edge, and in the zone immediately surrounding it, something strange had happened. The Staphylococcus colonies were transparent, colorless, dissolving. The bacteria were dying.

His laboratory assistant, V.D. Allison, was present. According to Allison, Fleming’s immediate response was not an exclamation of triumph. It was a quiet, puzzled, understated observation: “That’s funny.”

I’ve been thinking about those two words for a while. They tell you something important about how Fleming’s mind worked. Not “eureka.” Not “I’ve got it.” Just: that’s funny. The response of a person who has noticed something that doesn’t fit and wants to understand it before deciding what it means. There’s no performance in it. There’s just genuine, unmediated curiosity — the specific kind that makes a scientist look more carefully at a ruined experiment rather than throwing it away.

This is what I try to hold onto when I’m working through a bug or a result that doesn’t behave the way I expected. The instinct is usually to be frustrated, to attribute the anomaly to error, to move past it. Fleming’s instinct was to stop and ask: why is this happening? That two-word response — “that’s funny” — is, in some sense, the entire scientific method in miniature. Something unexpected appears. You notice it. You ask why.

Fleming studied the plate. He grew more of the mold. He tested its “juice” — as he initially called it — against a range of bacteria and found it effective against specific gram-positive pathogens responsible for scarlet fever, pneumonia, meningitis, and diphtheria. He tested its toxicity on a mouse and a rabbit and found it harmless. He named the active substance “penicillin,” published his findings in June 1929, and presented them to colleagues.

His colleagues showed almost no interest.

The Paper Nobody Read — For Eleven Years

This is the part of the story I struggle with most, because it’s not just historically frustrating — it has a specific human cost that I find genuinely difficult to absorb.

Fleming’s 1929 paper was scientifically clear and accurate. It wasn’t ignored because it was poorly written. It was ignored partly because Fleming couldn’t isolate the active ingredient in stable form — the chemistry required to purify penicillin was beyond what his laboratory at St. Mary’s could do — and partly because the scientific community simply wasn’t thinking about bacterial infection in a way that made his observation feel actionable.

But here’s the thing that I keep returning to: Fleming himself seems not to have fully understood what he’d found. For years after the 1929 paper, he used penicillin primarily as a laboratory tool for selectively inhibiting certain bacteria while culturing others. A useful technique, but not the therapeutic application that would eventually make it famous. He didn’t inject infected animals with the mold extract to see if it could cure them — an experiment that would have demonstrated its power immediately. Some historians suggest this was because his fundamental belief — that healing comes from within the body, not from external agents — made it hard for him to conceptualize a mold substance working as a systemic medicine.

I find this strangely relatable, in a slightly uncomfortable way. How many times have I worked on something, gotten an interesting result, noted it, moved on, and only much later — if ever — gone back to ask what it might actually mean? There’s a version of Fleming’s story where the failure isn’t in his laziness or his lack of rigor. It’s in a failure of imagination about what the observation was pointing toward. He saw the thing. He recorded the thing. He just couldn’t quite see the thing.

The gap between observation and application stretched from 1929 to 1940. During those eleven years, people with infections caused by the bacteria Fleming had demonstrated penicillin could kill continued to die of those infections. Wounded soldiers. New mothers. Children with scarlet fever. The treatment existed, in principle. Nobody had taken the next step to turn it into one.

The People Who Actually Made It a Drug

In 1938, Howard Florey, a pathologist at Oxford, was leafing through back issues of scientific journals looking for interesting research topics. He came across Fleming’s 1929 paper. He found the observations intriguing and brought the paper to Ernst Chain, a biochemist who had fled Germany as a Jewish émigré and was now working at Oxford. Chain, brilliant and abrasive in roughly equal measure, became convinced penicillin was worth pursuing seriously.

What followed was some of the most unglamorous and genuinely difficult science I’ve ever read about. Florey’s team began growing the mold in whatever vessels they could find — baths, bedpans, milk churns, food tins — and extracting the antibacterial compound from the culture broth. The yields were tiny. The substance was unstable. It degraded rapidly. The process of concentrating it was, in the words of one manufacturer who later attempted to scale it up, “murder.” The team employed a group of women — referred to in historical records as “penicillin girls,” at £2 per week — to inoculate and manage the fermentation vessels around the clock.

I keep thinking about those women. They don’t appear in the popular version of the penicillin story. They were paid £2 a week at a time when the average skilled worker in Britain earned perhaps £5. They were doing repetitive, precise, physically demanding work under wartime conditions, contributing directly to what would become one of the most important medical breakthroughs of the 20th century. They are not mentioned in the Nobel citation. Their names are mostly not recorded. I’m not sure what to do with that, except to note it and feel the familiar unease of recognizing that the people who did the necessary unglamorous work rarely appear in the stories we tell about the discoveries they made possible.

The Mouse Experiment That Changed Medicine

In May 1940, after months of work, Florey’s team finally had enough purified penicillin to run a proper animal trial. They injected eight mice with lethal doses of Streptococcus bacteria. Four received penicillin treatment; four received nothing.

The next morning, Florey came in early to check the results. The four untreated mice were dead. All four treated mice were alive, healthy, and moving normally.

Florey reportedly said, very quietly: “It looks like a miracle.”

I’ve read about this moment probably half a dozen times, and it still produces something in me that I can only describe as a kind of vertigo. Eight mice. The simplest possible trial. And it confirmed, with absolute clarity, what Fleming’s observation had suggested eleven years earlier but nobody had taken the next step to test: that this substance, derived from a mold that had floated up a stairwell into an uncovered petri dish during a London cold snap in 1928, could keep a living creature alive against an infection that would otherwise kill it. The distance between that petri dish and those eight mice — eleven years, multiple continents, bedpans full of mold broth, women paid £2 a week — was the distance between an observation and a fact.

In 1941, the first human trial took place. A police constable named Albert Alexander was near death from a severe infection — a scratch from a rosebush had spread to his face, scalp, eye, and lungs. Florey’s team injected him with their crude penicillin preparation. Within 24 hours, he was dramatically better. Treatment continued for five days, during which he improved steadily. Then their supply ran out. They tried extracting the drug from his urine to recycle it. It wasn’t enough. Albert Alexander died.

I don’t have a way to make this detail comfortable, and I don’t want to. The first person to respond to penicillin — who might have been the first person saved by it — died because there wasn’t enough of the drug to finish the job. That is simply what happened. It sits alongside the triumph of the mouse experiment as a reminder that the distance between “it works” and “we have enough of it” is not a footnote. For Albert Alexander, it was everything.

The Cantaloupe That Saved the World

Getting from “it works in mice” to “it works in sufficient quantities to treat humans at scale” turned out to be one of the most difficult production problems in medical history.

Florey brought the problem to the United States in 1941, working with researchers in Peoria, Illinois. They found that adding a byproduct of corn starch processing to the mold broth dramatically increased penicillin yields. They also launched a global search for strains of Penicillium mold more productive than Fleming’s original.

The winning candidate was found not in a laboratory but in a supermarket. A researcher named Mary Hunt — nicknamed “Moldy Mary” by her colleagues — was tasked with collecting rotting produce from local markets to test for high-yield mold strains. One afternoon she brought in a cantaloupe covered in a particularly promising golden mold. Tests showed it produced six times more penicillin than Fleming’s original strain. This cantaloupe mold became the basis for virtually all subsequent penicillin production worldwide.

Every time I come back to this detail, it does something to my sense of where important things come from. The greatest medical breakthrough of the 20th century traces back, through a chain of mold cultures and bedpans and women paid £2 a week and a mouse experiment and a forgotten journal paper, to a rotten piece of fruit in an Illinois supermarket that a woman with a very good nickname had the presence of mind to bring in and test. I don’t know what the opposite of a romantic origin story looks like, but I think it might look something like this. And I mean that as a compliment.

By 1945, penicillin was being mass produced. It is estimated to have saved over 500 million lives since its discovery. It transformed warfare — for the first time in history, more soldiers in World War II died from combat injuries than from infection, reversing a ratio that had held through every previous major conflict.

Who Actually Gets Credit?

In 1945, Fleming, Florey, and Chain received the Nobel Prize in Physiology or Medicine. Norman Heatley — the biochemist who figured out how to extract and purify penicillin in sufficient quantities, without whom the clinical trials couldn’t have happened — was not included. Oxford made up for this partially in 1990 by awarding Heatley an honorary doctorate of medicine, the first such honorary degree in the university’s 800-year history. He received it 45 years after the men who received the Nobel.

Fleming became, through the specific mechanism of press coverage, the famous face of penicillin. He was consistently gracious about this, consistently modest, consistently pointing credit elsewhere: “I did not invent penicillin. Nature did that. I only discovered it by accident.”

Florey, whose team had done the scientific heavy lifting — the purification, the animal trials, the human trials, the production scale-up, the wartime logistics — received considerably less public recognition. He was reportedly quietly furious about this for the rest of his life, in the restrained way of someone with too much dignity to complain loudly about something genuinely unjust.

I’ve been on teams where credit felt wrong after the fact, and I recognize the specific flavor of frustration Florey seems to have carried. It’s not exactly anger. It’s something more like the persistent, low-level awareness that the record says something different from what you know to be true. You can’t correct it easily without seeming self-aggrandizing. So it just sits there.

A Thought to Leave You With

In his Nobel acceptance speech in 1945, Fleming said something that has not aged well — or rather, has aged too well, in the way predictions do when nobody heeds them.

He warned, publicly and explicitly, that misuse of penicillin might lead to resistant bacteria. He described a specific scenario: someone treats themselves with too little penicillin to kill their infection, allowing resistant bacteria to survive and multiply. He said this in 1945, before the drug was even widely distributed.

We did not listen well enough. Antibiotic resistance is now one of the most serious public health threats in the world. The WHO estimates that drug-resistant infections already cause over a million deaths annually.

I keep coming back to that timeline. Fleming discovered penicillin in 1928. He warned about resistance in 1945. The resistance crisis he described is now a present-day emergency. That’s nearly a century from observation to consequence — long enough that we’ve told the heroic origin story a hundred times in the interval and perhaps forgotten to keep telling the warning that came with it.

There’s a particular kind of frustration that comes with warnings that arrive clearly and are understood clearly and are then gradually, collectively, set aside until the thing predicted actually happens. I think about it sometimes when I’m reading security bulletins that describe vulnerabilities nobody has quite gotten around to patching yet. The mechanism is different. The shape of the problem is the same.

“That’s funny,” Fleming said, looking at his ruined petri dish in 1928. He was right. It was funny. It was also the beginning of something that saved hundreds of millions of lives — and came with a warning we are still, eighty years later, working out whether to take seriously.

How Fleming’s Forgotten Petri Dish Accidentally Discovered Penicillin in 1928