Penicillium chrysogenum

by Rachel Adams

Penicillus means “little brush” in Latin, and if Penicillium is a paintbrush, then the world is its canvas.

This mold is everywhere. The tips of the paintbrush produce a spatter of tiny spores that become airborne easily and speckle practically the entire globe, not just indoors. There are sixteen species of Penicillium that are regulars in homes and other buildings, and perhaps the most common is Penicillium chrysogenum.

That blue-green fuzzy mold that got to your lemon or slice of bread before you could? That was probably P. chrysogenum. Besides appearing on your kitchen counter, this fungus may also leave a mark in your medicine cabinet, for it produces the antibiotic penicillin.  The fact that it is such a common tabletop mold as well as a producer of the first widespread life-saving drug against bacteria may make it the second most influential fungus in human history.* As John Pitt wrote in 1979, “Rare indeed must be the human individual who has not encountered fungi of the genus Penicillium or been affected by one of the many metabolites [compounds] produced by them.”

Reading up about Penicillium, I couldn’t help but wonder, are those two linked? Is the fact that Penicillium is everywhere somehow related to the fact that it makes such a potent weapon against some bacteria?

P. chrysogenum and other Penicillium grow really well in culture, the traditional way of tracking what fungal spores are present but invisible. The ample spores germinate easily, and when they do start to grow, they grow fast. These features actually had a hand in making it easier for us – or more precisely, for Alexander Fleming – to happen upon its second notable feature, its antibiotic properties.

The story is that Fleming was cleaning house (or really, “cleaning lab”) upon returning to work from a holiday. In his absence, many of his cultures of bacteria had become contaminated with intruders from the environment. On one culture, he noticed that the contaminant had repelled the growth of the bacterium that he’d been trying to grow.  The (in these case, welcomed) intruder turned out to be the fungus Penicillium, and the antibiotic compound later named penicillin, which thanks to the work of other scientists such as Howard Florey, Ernst Chain, and Norman Heatley, became the lifesaving drug. (Fleming’s original strain of fungus is now known to be Penicillium rubens, closely related to P. chrysogenum; both species produce the drug.)

Is P. chrysogenum so common because it makes penicillin, giving it an edge over its bacterial competitors? Probably not.

For one, Penicillium is just one of many fungi that are common in the air. While culture-based work would suggest it is the most common, my and other’s work surveying the indoor environment using DNA-based detection techniques indicates that its only in the top ten. More importantly, though, many fungi make compounds that can be deadly to bacteria, fungi’s main microbial competitor for something to eat and a place to live. In fact, fungi make many compounds that are harmful to other organisms, compounds known as mycotoxins. For example, Penicillium griseofulvum makes the drug griseofulvin that is used to combat another fungus, one that causes ringworm; amatoxins in the Amanita mushrooms can cause stomach trouble and even death in humans. Penicillin belongs to just one of many different classes of antibiotics made by fungi; the reason we are able to exploit it is because it happens to not be too toxic for use in human therapy.

So, while P. chrysogenum is both extremely widespread and produces the famous penicillin, we cannot say it’s widespread because it has strategies for defeating its rivals. For the most part, why some species are very successful remains left to speculation.

Usually only one of those features of P. chrysogenum – the drug-making one – is considered beneficial to humankind, but I would argue that it being common also does us a favor. The principal role of P. chrysogenum in nature is to break down organic material, to decompose products into their basic elements. Sometimes we may wish it weren’t so good at its job, but ultimately we do greatly benefit from the recycling of organic material – lest we be buried under plant products.

In that way, the main creative outlet for the little brushes of Penicillium chrysogenum is a true art: the art of destruction.

* The first, of course, must surely be Saccharomyces cerevisiae, the yeast that brings us bread and alcoholic beverages. Interestingly, a non-random poll of my labmates yielded agreement on 1st and 2nd place. 3rd place was more contentious.


The genus Penicillium, by John I. Pitt. Academic Press, London, 1979.

Microorganisms in Home and Work Environments: Diversity, Health Impacts, Investigation and Control (2nd Edition), edited by Brian Flannigan, Robert A. Samson, & J. David Miller. CRC Press, 2011. [photo credits]

About the Author

Rachel Adams is a post-doctoral fellow in the Department of Plant & Microbial Biology at the University of California at Berkeley. She studies the dispersal of fungi in homes and tweets at @Rachel_I_Adams.