Turns out bacteria cells might not actually outnumber your own, but there's still a heck of a lot of the little buggers living all over you. And since you're already sharing the rest of yourself with your partner—your bed, your shower, your saliva—it makes sense that you'd share bacterial colonies too.
And isn't that kind of beautiful?
This communal bacteria news comes courtesy of a recent study in mSystems, an open access journal published by the American Society for Microbiology. It compared bacterial profiles from 17 different spots on the human body for a group of cohabiting couples. Spots included the soles of the feet, the outer nostril, the eyelid, the bellybutton, the inner thigh, and the armpit. Thankfully the participants themselves did the swabbing, so the microbiologists didn't have to swab anyone's bellybutton gunk. This seems like a less accurate way to do the study, since the average person might not be a great swabber, but I suppose if I had to choose between wiping q-tips on strangers and just asking them to do it themselves I would probably spare myself too.
After the swabbing, the microbiologists set about creating a computer model to figure out which areas had the most similarity between partners. You might think it was something intimate, like maybe the torso or even the thigh, but it was in fact the feet, which is also one of the two most diverse places on your body (bacterially speaking). Your feet, eyelids, and back are the spots with the most microbial similarity to your partner. Having foot similarities with a person you're living with makes sense: You walk on the same floors and shower in the same stall. The eyelids are a bit of a mystery, but hey, the data speak for themselves.
Thighs actually had very little in common from partner to partner. The bacteria living there are more similar to those found on other members of your sex than on people you're sexing. Of course, as is the case with most studies on romantic couples, all of the participants involved were heterosexual. It would have been interesting to see whether same-sex couples share more microbial communities, but evidently that wasn't of interest to the researchers.
All of these findings are perfectly in line with past findings on microbiomes, namely that we share our little buddies with everyone and everything—and as much as possible. You and your partner have similar mouth colonies, which makes sense given that a 10 second kiss transfers 80 million bacteria. And that may be part of how your immune system comes to look a lot like your partner's over time.
When you arrive in a hotel room, you colonize that room with your personal bacterial community within hours. And while you're away from home, your contribution to that microbi-home (sorry) diminishes the longer you're gone. You even share a microbial profile with your dog.
Nearly every choice you make in your life affects your bacteria. What you eat, where you live, how you bathe—it all adds up. If you drink a lot of alcohol, your skin probably harbors a microbe called Brevibacterium, which may be feeding off the ethanol you secrete in your sweat glands. Owning pets and going outside for more than four hours a day supplements your microbiome significantly. If you have allergies, you probably have more diverse bacteria in and around your nose.
So in a way, your bacteria reflect who you are as a human being. Slowly falling in love isn't just about getting to know someone, it's also about slowing sharing more and more of your microbiome with them. Is that not the stuff of romance novels?