The discovery of Penicillin

Welcome! I am excited to explore the incredible story of penicillin with you. Before we dive into the history, I have a quick question to help me tailor this session specifically for you.

Are you exploring the discovery of penicillin for a history or science class, personal curiosity, or something else?

Just personal curiosity

It is wonderful to have you here! Let's travel back in time to the early 20th century, long before we had a quick pill to cure everyday sicknesses, to see just how different—and dangerous—the world used to be.

The World Before Antibiotics

Imagine living in a world where a simple scratch from a rose thorn while gardening, or a minor scrape on the playground, could be a literal death sentence. Before the discovery of penicillin, humanity was largely defenseless against microscopic invaders. Doctors had incredibly limited options to fight infections, often resorting to like carbolic acid, or even toxic chemicals containing arsenic.

To understand the sheer scale of the danger, let's take a look at the mortality rates for some common, everyday ailments before antibiotics changed medicine forever.

Infection Type	Pre-Antibiotic Era Survival Rate	Key Cause
Strep Throat / Scarlet Fever	Often fatal or led to heart damage	Streptococcus pyogenes
Wound Infections	High amputation and death rates	Staphylococcus aureus
Bacterial Meningitis	Over 90% mortality rate	Various bacterial species
Pneumonia	Roughly 30% mortality rate	Streptococcus pneumoniae

Because of this, hospital wards were filled with patients waiting for their bodies to either miraculously fight off an infection or succumb to it. This high-stakes reality set the stage for one of the most famous accidents in scientific history.

Continue to Fleming's accidental discovery in 1928

Let's fast-forward to September 1928, when a messy desk and a well-deserved vacation combined to change the course of human history.

The Messy Lab of Alexander Fleming

When Scottish bacteriologist Alexander Fleming left his lab at St. Mary's Hospital in London for a two-week family holiday, he wasn't trying to invent a miracle drug. In fact, he left behind a notoriously cluttered workbench. Among the clutter was a stack of petri dishes where he had been growing , the nasty microbes responsible for painful boils, sore throats, and deadly blood infections.

Upon returning to his lab on September 3, 1928, Fleming began sorting through his dirty dishes to clear them for sanitization. As he examined the plates, one particular dish caught his eye. A stray spore of a blue-green mold—later identified as —had drifted in through an open window or up from a dusty hallway, landing right in the middle of his bacterial colony.

What Fleming saw next was a true revelation. Instead of the mold and bacteria happily growing side by side, there was a clear, glassy ring of empty space around the mold. The bacteria closest to the mold had literally dissolved. Fleming famously noted that the mold was producing a substance that killed the staph, which he initially called mold juice before officially naming it penicillin.

Alexander Fleming's petri dish showing a zone of inhibition around Penicillium mold.

I've started generating an interactive visualization of Fleming's contaminated petri dish so you can see exactly how the mold cleared out the bacteria, leaving a distinct ring where nothing could survive.

Fleming quickly set to work testing this mold juice and made some crucial observations. He discovered that even when he diluted the liquid 800 times, it was still highly effective at stopping bacteria. Even better, when he injected it into rabbits and mice, it was completely nontoxic, unlike the harsh carbolic acid of his day. However, he hit a massive roadblock when he tried to isolate and purify the active chemical. It was highly unstable, breaking down almost immediately in his lab, leaving him unable to produce enough of it to actually treat a human being.