# Penicillin mould (Penicillium rubens)

**Penicillium rubens* (Fleming’s “*P. notatum*”)*

**Known today as.** A blue-green mould of bread, fruit, and damp.

Kind: Fungus

## What was it used for?

Folk healers laid mouldy bread on wounds long before anyone knew why.

## What does modern science say?

The first antibiotic, and one of the most important medicines in history.

## Pharmacognosy and toxicology

- **penicillin G**: benzylpenicillin
- **6-aminopenicillanic acid**: the core of the semi-synthetic penicillins

**Mechanism.** Penicillin binds the enzymes that cross-link the bacterial cell wall, so the dividing bacterium bursts; it has no target in the human cell, which is why it is so safe.

**Dose.** Allergy, in up to a tenth of those who report it, is the only common harm; anaphylaxis is rare.

**Standing.** Benzylpenicillin is on the WHO list of essential medicines; Fleming’s observation of 1928 and the Oxford work of Florey, Chain, and Heatley gave the drug by 1941.

## What lore surrounds it?

In 1928 Alexander Fleming noticed that a stray mould had killed the bacteria around it on a culture plate. Howard Florey and Ernst Chain made penicillin a medicine in the 1940s, and the three shared the Nobel Prize in 1945.

## Sources

- Fleming, A. (1929). On the antibacterial action of cultures of a Penicillium. *British Journal of Experimental Pathology* 10: 226–236.
- Fleming, A. (1929). “On the antibacterial action of cultures of a *Penicillium*.” *British Journal of Experimental Pathology* 10.
- Chain, E., et al. (1940). “Penicillin as a chemotherapeutic agent.” *Lancet* 236.

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*Penicillin mould.* In *Materia Medica*, edited by B. C. P. Statema. Version 1, revised 2026-09-25. The Thinking Press. https://thethinkingpress.org/materia-medica/penicillium

Historical description, not medical advice.