How Mold Led to the First Antibiotic

Black-and-white photograph of a scientist in a lab coat holding up a petri dish for observation. The scientist appears to be examining bacterial cultures in a laboratory setting.

Did you know that the antibiotic penicillin comes from mold? Penicillin works by destroying bacterial cell walls, making it effective against many infections like strep throat and pneumonia.

 

EARLY DISCOVERIES

 

In 1896, Italian scientist Bartolomeo Gosio discovered that a mold called Penicillium glaucum could stop the growth of deadly bacteria like anthrax. He even managed to isolate the active compound, later known as mycophenolic acid — the first antibiotic ever identified. At the time, few realized how significant his finding was, and it soon faded into obscurity.

 

THE FLEMING STORY

Decades later, other researchers observed similar effects from different molds but didn’t pursue them further. You may have heard about Alexander Fleming’s famous petri dish. In 1928, the Scottish scientist noticed something remarkable. While studying bacteria in his London lab, he found that one of his petri dishes had been contaminated by a patch of blue-green mold. Around that mold, the bacteria had completely dissolved.

The mold — later identified as Penicillium notatum — was releasing a natural substance that stopped bacteria from multiplying. Fleming called it penicillin. He discovered that the substance was not toxic to animals and could fight a wide range of dangerous bacteria, including those causing strep throat, pneumonia, and meningitis.

Black-and-white photo of Alexander Fleming’s petri dish showing staphylococcal bacteria colonies dissolving near a Penicillium mold colony, demonstrating the discovery of penicillin. (Source: British Journal of Experimental Pathology, 1929)
Photograph of a culture plate illustrating the breakdown of staphylococcal colonies surrounding a Penicillium mold colony. Adapted from the British Journal of Experimental Pathology (1929, Vol. 10, pp. 226–236).

 

SCIENTIFIC IMPACT

 

He published his results in the British Journal of Experimental Pathology, but at the time, few paid attention. For over a decade, his work went largely unnoticed. It wasn’t until scientists Howard Florey, Ernst Chain, and Norman Heatley revisited his findings in the late 1930s that penicillin was purified and produced for real-world use. The results changed modern medicine forever. Today, penicillin and related antibiotics remain some of the most widely used treatments for bacterial infections.

 

Still, the real genius wasn’t human invention, it was nature’s chemistry. Molds had been creating antibacterial compounds for millions of years to defend themselves and balance ecosystems. Fleming’s true achievement was recognizing what nature had already designed: a microscopic system of resilience that humans had simply overlooked.

 

 

References:

Giannis, A. (2025). Pearl Necklaces, Molds, Asthma, and a Discovery of the Century (Penicillin). In: Natural Compounds Enabled Therapies. Springer, Cham. https://doi.org/10.1007/978-3-031-94024-8_8.