by Veronique Greenwood
In 1963, two dermatologists at the University of Pennsylvania performed a peculiar experiment with sweat. They sucked it up from subjects’ skin with a number of very fine glass tubes. They then planted a different bacterium normally found on skin in each tube. They sealed the tubes with hot wax, gave the organisms 24 to 48 hours to enjoy themselves in the fluid, and then cracked the tubes open. And here is the weird part, or at least the first weird part. When they opened the tubes, the first thing they did was pause to take a long sniff of each one. They were trying to see if any of the tubes smelled like stinky sweat.
The goal of the experiment was to see which bacteria known to live quietly on our epithelium were responsible for the smell of sweat, which by itself has long been known to be odorless. Your sweat does not stink, and neither does anyone else’s. It is the bacteria. Albert Kligman, one of the scientists who pioneered treatments for acne but whose controversial experiments on prisoners sparked nationwide clinical trial ethics reforms, had previously written that all skin bacteria could produce the smell. But when the dermatologists sniffed their tubes, they found Kligman’s assertion to be wrong. They wrote, “Only the gram positive resident organisms, coagulase negative Staphylococci and diphtheroids, which completely dominate the axillary microflora, generated the typical odor in an unequivocally sharp fashion.” Or in ordinary English, only the Staphylococcus bacteria and diptheroid bacteria, which are super common and abundant in armpits, were stinky.
In recent years, molecular biologists have been able to get even more specific. They’ve identified several molecules responsible for the pungent scent of body odor, and have found that one in particular, 3-methyl-2-hexenoic acid, starts out as a slightly longer molecule in sweat. An enzyme from those “diphtheroids,” a name for certain bacteria from the genus Corynebacterium, slices off the end of that odorless precursor. The leftover molecule is what makes pits, and sweat more generally, foul.
The evolutionary relationship between humans and these sweat-munching microbes is probably quite old. I’ll speculate here that other primates also play host to species of Corynebacterium that, in turn, account for their unique odors. The Corynebacterium have evolved with us. And, our bodies evolved with them. The enzymes of Corynebacterium recognize specific precursor structures on the biochemical products our armpits produce. What circumstances made this interaction favorable, or at least not harmful, to either party is not clear, though researchers sometimes speculate that body odors produced by bacteria may be more important in human courtship and mating than we think. So go ahead and sniff the one you love, or, rather, sniff the ones who live on the one you love, who are very likely to be Corynebacterium.
Natsch, A., H. Gfeller, P. Gygax, J. Schmid, and G. Acuna. 2003. A specific bacterial aminoacylase cleaves odorant precursors secreted in the human axilla. J. Biol. Chem. 278:5718-5727’
Shehadeh, N. and Kligman, 1963. The bacteria responsible for apocrine odor, Part II. J. Invest. Dermatol. 41:1-5.
About the Author
Veronique Greenwood is a former staff writer at DISCOVER Magazine. She writes about everything from caffeine chemistry to cold cures to Jelly Belly flavors, and her work has appeared in Scientific American, TIME.com, TheAtlantic.com, and others. Follow her on Twitter here.