When you’re in a relationship (especially when children are involved) you share everything, whether you like it or not. Boundaries are for single folk. So it came as little surprise when a recent study found that cohabiting, sexually active couples share billions of bacteria. Spouses host communities of microbes on their feet, torsos, and eyelids. These communities can be understood as twin cities or as one big happy microbial family.
“The most surprising aspect of the study was that we could identify a microbial fingerprint common to cohabiting couples,” coauthor on the study Josh Neufeld of the University of Waterloo told The New York Times. Neufeld’s study involved only 20 participants (10 couples), but involved 330 skin swabs collected from 17 body parts of each participant. Computer algorithms managed to match couples based on their microbes alone, with 86 percent accuracy. This is all to say that every happy couple is also a bacterial breeding ground.
Here’s how all that multiplication takes place.
Not My Feet, Our Feet
When Neufeld and colleagues analyzed all 20 microbial communities (mostly harmless bacteria that inhabit human skin) they found that 35 percent of the couples surveyed had microbial communities on their feet that were more similar to that of their partners’ feet than anyone else’s feet. That’s seven-fold higher than the five percent expected by chance, strongly suggesting that couples influence the microbial communities living on each other’s feet. Ditto for bacteria found on the couples’ torsos (21 percent), navels (20 percent), and eyelids (17.5 percent).
Coauthor Ashley Ross, also of Waterloo, told The New York Times this was probably because sharing a shower or walking around barefoot are great ways to pick up any of the one million biological particles humans shed every hour. As for the torso, navel, and eyelids, Ross chalks up the similarities to sharing beds and nuzzling common pillows.
The researchers found few similarities between the bacteria living in couples’ noses, which isn’t terribly surprising seeing as most couples don’t spend much time up each other’s nostrils. But they also found virtually no similarities between the bacteria residing on couples’ thighs. Which is a bit more surprising. Because, you know.
Sharing Meals and Microbes
Prior studies have shown that the similarities between your microbiome and that of your significant other are not only skin deep. We also share gut microbes with our close associates, including family members and pets, and one large study of blood samples from 670 people found that couples and families even share similar immune cells. That’s probably because couples and families tend to have similar diets, lifestyles, and exposures to viruses and environmental toxins. It’s all part of a phenomenon known as spousal concordance—we pick up the habits of the people closest to us, from their drinking habits to their stress management skills.
Our bodies tend to reflect that—microbes, and all.
Individuality is All About Thighs
The reason why couples’ thighs didn’t match up is actually fascinating. Because there’s actually even more powerful influence over your microbial community than with whom you’re sleeping—your biological sex.
The women in the study were more than 1,000 percent more likely to host Lactobacillus bacteria on their thighs than men, and men were 300 percent more likely to host Allolococcus bacteria on their thighs than women. In fact, the microbial differences on the thighs were so unique that computer algorithms could differentiate between men and women based on thigh bacteria alone with 100 percent accuracy. Ross suspects these strong differences are influenced by the community of bacteria within the vaginal microbiome—a microbiome so unique that it has its own consortium.