Many people searching for the origin of penicillin look for information on a “penicillin plant.” This search is understandable, as many medicines are derived from plant-based sources. The story of penicillin begins with a natural organism, but its identity is often a point of confusion. This antibiotic does not originate from the leaves, roots, or flowers of a plant, but from an accidental discovery involving a different kind of life form.
The True Source of Penicillin
The source of penicillin is a fungus, a mold called Penicillium. In September 1928, Scottish physician Alexander Fleming returned to his laboratory at St. Mary’s Hospital in London. He was sorting through petri dishes where he had been growing Staphylococcus bacteria, responsible for infections like boils and sore throats.
Fleming noticed on one open dish that a blob of blue-green mold had grown. In the area immediately surrounding the mold, the bacterial colonies had been destroyed. He realized the mold was producing a substance that was lethal to the bacteria.
This “mold juice,” as he called it, was capable of killing a wide range of harmful bacteria. Fleming identified the mold as a strain of Penicillium notatum, now reclassified as Penicillium rubens. This mold is visually similar to common molds on spoiled bread or citrus fruits.
This accidental finding marked the beginning of the antibiotic era. Fleming published his research, but it was nearly a decade before a team at the University of Oxford, led by Howard Florey and Ernst Chain, could purify penicillin. Their work, spurred by the need for treatments for infected wounds during World War II, turned Fleming’s discovery into a mass-produced drug.
Fungi Are Not Plants
The common misconception of a ‘penicillin plant’ stems from a misunderstanding of biological classification. Fungi, the group that includes mushrooms, yeasts, and molds like Penicillium, belong to their own distinct kingdom. For many years, fungi were classified with plants because they are stationary and have rigid cell walls. Modern science has shown they are fundamentally different and more closely related to animals than to plants.
The most significant difference lies in how they obtain energy. Plants are autotrophs, meaning they create their own food through photosynthesis. Fungi are heterotrophs and must absorb nutrients from their environment. They do this by secreting digestive enzymes that break down organic matter.
Another defining difference is the composition of their cell walls. Plant cell walls are primarily made of cellulose, a complex carbohydrate. In contrast, the cell walls of most fungi are made of chitin, a durable polymer that is the same material found in the exoskeletons of insects and crustaceans.
From Mold to Medicine
Transforming the substance produced by a common mold into a safe and effective medicine is a complex industrial process. Scientists do not simply scrape mold from a surface; instead, they cultivate massive quantities of a high-yielding Penicillium strain in controlled environments. This process, known as fermentation, takes place in enormous stainless-steel tanks that can hold tens of thousands of gallons.
Inside these fermenters, the fungus is provided with a carefully prepared liquid diet, a nutrient-rich “broth” containing sugars like lactose, nitrogen sources, and minerals. The environment within the tank is meticulously controlled; temperature, pH levels, and oxygen are constantly monitored and adjusted to encourage the fungus to produce the maximum amount of penicillin. The fungus secretes the penicillin into the surrounding broth as a secondary metabolite, meaning it is produced after the organism has completed most of its growth.
Once the fermentation cycle is complete, the process of extraction and purification begins. The broth is filtered to separate the liquid, which now contains the penicillin, from the fungal mass. The penicillin is then extracted from the broth using a solvent and undergoes several purification steps to create a stable, concentrated product. This pure penicillin is crystallized into a powder, which can then be formulated into the various forms of the antibiotic used in medicine today.