Propionibacterium acnes

by Courtney Humphries

In sixth grade, the boy I had a crush on pointed at me and called me “Fuji face” in front of a group of classmates. As in Mount Fuji. I hurled back an insult, but couldn’t remove the sting of the image: my skin erupting with volcanoes.

I had developed acne at an early age. My skin was in revolt, and like many situations in adolescence, I had no control over it. Each morning I trudged to the bathroom mirror, afraid of what new geological features had surfaced on my cheeks, nose, or forehead overnight.

According to the theory at the time, my skin had been hijacked by a microbe called Propionibacterium acnes—the same bacterium responsible for inflaming the skin of 80 percent of American adolescents and teens. Some bacteria cause disease and death, but P. acnes inflicted a different kind of misery: it devastated adolescence, destroyed confidence, and laid social lives to waste.

Like many acne sufferers, I slathered my skin with antibacterial creams and popped oral antibiotics prescribed by my dermatologist. Although they never worked very well for me, I took them dutifully because research had shown that culling the numbers of P. acnes would result in clearer skin. But as any acne sufferer can tell you, there are no simple solutions. Acne is not a typical bacterial infection, and to this day the role of P. acnes in the condition is unclear. Are we unfairly blaming our pimples on a microbe?

The notion that acne was caused by bacteria was first proposed by a German dermatologist named Paul Gerson Unna. His 1896 book The Histopathology of the Disease of the Skin describes every type of pustule, papule, nodule, and cyst in oily microscopic detail (he even debunked a myth at the time: that blackheads were black because they contained specks of coal dust). Every zit contained “a swarm of micro-organisms,” he found, but what attracted his attention was a particular organism deep within pimples—“small and plump” rod-shaped bacteria arranged in threads of three or four. He considered it exceedingly likely that these were the cause of acne.

The bacteria were eventually called Bacillus acnes, Corynebacterium acnes, and later P. acnes. Despite being linked to acne for well over a century, however, their role in provoking the inflammation of acne is still debated.

Here’s what we do know. P. acnes is a rod-shaped bacterium that lives in the sebaceous glands, which are embedded in hair follicles and produce an oily, stick substance called sebum. P. acnes happens to feed off oils and fats, and as Unna noted, “there could scarcely be a better protected and tempered chamber” for fat-digesting bacteria than marinating in the depths of an adolescent’s follicles. It uses a fermentation process that produces fatty acids as a byproduct, including propionic acid for which it’s named. Though known for its home in the skin, it’s also found in the urinary tract, mouth, and large intestine. In addition to acne, it’s been linked to other inflammatory conditions of the skin, lungs, heart, and eyes.

According to the general view, acne seems to result from several factors, only one of which is P. acnes. Other factors are large sebaceous glands, overabundant hormones that boost sebum production, and skin follicles jammed with excess cells. P. acnes produces chemicals that irritate the follicles and cause an inflammatory reaction. The result: skin volcanoes.

Here’s the catch: although large numbers of these bacteria are found in acne-affected skin, they are also found in skin that is smooth and unblemished, and more bacteria do not always equal more severe acne. P. acnes is not so much a disease-causing pathogen as a typical resident of skin that that creates a bad reaction in certain conditions. Acne runs in families, which suggest that genetic factors are just as important, if not more so, than bacteria.

Recently, two dermatologists at Cardiff University have gone farther: They suggest that acne has nothing to do with P. acnes at all, based on analysis of inflamed acne lesions. They believe it is simply an innocent bystander.

Why is it important to determine whether P. acnes is a friend or foe? For decades, adolescents and adults worldwide have been taking antibiotics in the hope that battling this microbe will improve their skin, which has not only created antibiotic resistance in P. acnes, but contributes to the larger problem of resistance in other microbes. Now that we know how important our resident microbes are, overmedicating against a bacterium that may not even be an enemy seems like waste—and even dangerous. As much as I’d like to blame my adolescent strife on this microbe, our relationship is more complex.

About the Author

Courtney Humphries is a freelance journalist and author in Boston, specializing in science, health, and nature. Her work has appeared in publications including the Boston Globe, Technology Review, Nature, Science, New Scientist, and Wired. She is the author of Superdove: How the Pigeon Took Manhattan….And the World, a natural history of pigeons published Smithsonian Books. She is a graduate of MIT’s Graduate Program in Science Writing.