What Do Microbes Have to Do With Your Love Life?

8 million microbes per second.

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"Dear Josephine, I am coming home in three days. Don't wash." —Napoleon Bonaparte

True love is such a uniquely human emotion. Or is it?

If you could see all the genes in your body, you'd be shocked. Only 1 percent of them are human. The other 99 percent belong to bacteria, yeast, viruses, and tiny protozoans. This group of microbes is called your microbiota, and it coats your entire body, inside and out.

Your gut microbiota, at about three pounds, is among the most crowded spaces on the planet. It is host to a thousand or so different species, munching together on the continuous buffet your gut obligingly provides.

When it comes to genes, we are vastly outnumbered. Amazingly, these microbes and their genes may affect our love lives, helping us to pick compatible mates like a biological dating app. Depending on the state of your microbiota, that can be a good thing or a bad thing.

Starting with flies

An inspired 2010 Israeli study took a batch of fruit flies and split them into two groups. They fed one group a diet featuring molasses, the other starch. They did this for two generations. Each group developed a suitable microbiota, perfectly tuned to digest their respective diets.

Then they mixed the groups together in foursomes—a pair from each group—in a mating chamber. It sounds plush, but a fly-mating chamber is really just a tiny, plastic bowl with a clear top so researchers can count the sex acts. Such is the life of a grad student.

Despite the total access between all the flies, they preferred to mate with those having the same microbiota.[1] For dozens of generations, the flies kept to their own group, essentially acting as two separate species. How do we know it was due to the microbiota, not just the smell of molasses on the breath? Simple: When the researchers gave the flies antibiotics to kill their microbiota, their preferences vanished.

According to lead scientist Eugene Rosenberg, "symbiotic bacteria can influence fly mating preference by changing the levels of… sex pheromones. The bacteria do not produce the sex pheromones, but regulate [their] production." Pheromones are somewhat mysterious chemicals that can act as sexual attractants. Insects will fly miles based on sniffing a single molecule of the stuff.

So microbes can alter the flies' production of sex attractants? That puts a serious amount of power into a puny microbe. It shows how important a microbiota can be to mating choices, at least for a fly.

Moving on to humans

In a follow-up paper, Rosenberg implies that flies aren't the only animals whose mate choice is manipulated by microbes: "Bacteria contribute to smell in humans and other animals, and smell is an important input in choosing sexual partners." Your bacteria convert skin oils into your own personal blend of fatty acids, which determines how you smell. It's your signature scent.

Rosenberg and team further speculate that antibiotic treatments in infants may affect this scenario, potentially leading to loss of bacterial groups and a different scent.[2] Could early antibiotic use affect your future love life? It's a disturbing speculation, but Rosenberg believes that by carefully repopulating the microbiota after antibiotic use, the condition might be reversed.

Microbes and hormones

Seen objectively, sex is sometimes a sweaty, messy affair, but somehow we manage to get past that. Thus the global population of 7 billion people. But even here, bacteria may play a role by manipulating our hormones.

Microbes communicate with each other using hormones, just like animals do.[3] These chemicals may help us to get in the mood for sex. Oxytocin, for instance, is involved with maintaining good health and fast healing, but it also plays a role in social bonding. Oxytocin is the "cuddle hormone," which may help us to ignore the inconvenience of sex and get down to the business of propagation.

This is also a big win for our microbes because sex helps them to find new human territory. Smooching can make their day: Intimate kissing can transfer 8 million bacteria per second between participants.[4]

Levels of oxytocin in your body can be enhanced by consuming probiotics, like Lactobacillus reuteri.[5] For lack of a better term, we could call these microbes "love bugs." Don't rush to buy supplements, though—you can boost your own levels of L. reuteri with high-fiber foods (more on this later).

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Sniffing out a compatible mate

Mate selection is also affected by our own immune systems, primed by microbes. The major histocompatibility complex (MHC) consists of immune proteins that give each of us a unique odor-print, on top of our microbial aroma. Your MHC proteins are created by exposure to microbes, representing the condensed version of a lifetime of battling pathogens.

The theory is that we unconsciously select mates who have completely different MHCs. That means they will complement our own, effectively doubling our resistance to pathogens. When you select a mate with a compatible MHC profile, they will smell good to you—and if you have kids, they will have a head start on a healthy microbiota.

Microbes affect mice, too

A bad microbiota, like those that cause gut problems, including IBS and IBD, can make you uncomfortable and unsociable. But a good microbiota can make you the life of the party. Mouse studies have shown just how important this effect is.[6] Mice are perfect subjects for microbe studies because you can raise them to be completely free of microbes, so-called germ-free mice.

Mice are typically gregarious, but germ-free mice are hermits, preferring an empty chamber to the parties going on elsewhere in the nest. Providing these recluses with probiotics cures them of their standoffishness.

We don't have germ-free humans to experiment on, but we know a bad gut can keep you housebound. A good gut, on the other hand, will improve your odds of getting out and meeting new people.

Slow but handsome

Here is one last example of how microbes can control your love life.

A parasite called toxoplasma that lives in cat poop can infect people. It can even grow in your brain, somehow evading the immune system. A Toxoplasma infection can boost levels of testosterone in men, making them taller and more masculine looking. However, Toxoplasma infections cause slower reaction times and may increase the odds of schizophrenia. It is, after all, a brain infection. So it is disturbing to report that women prefer infected to uninfected men.[7]

This is a sobering example of a microbe that controls our looks, behavior, and mate choice, and not necessarily for the better. Napoleon would agree: Looking good may not be as important as smelling good.

Now what?

How should you deal with this astonishing intrusion of microbes into your most intimate life choices? For one thing, don't drown your signature scent in too much perfume or cologne. Despite the commercials, deodorant is not a sex attractant. Some people, like Josephine Bonaparte, don't bathe that often. If you're adventurous, you could join the no-showering movement that asks us all to get used to a little BO to save water. See what you can get away with.

For another, make sure you have a good microbiota. That can help you have great skin, lustrous hair, and an irresistible scent of health.

To tune up your microbiota, follow these simple steps:

• Eat plenty of fiber-filled vegetables, including asparagus, artichokes, broccoli, and leeks. Fiber feeds your beneficial microbes, which then keep away pathogens.
• Eat dark-colored fruits, like cherries and blueberries, that contain anti-oxidants, helping you deal with inflammatory pathogens.
• Try fermented foods containing living microbes, like yogurt and kefir.
• Eat a little fish on occasion; it has anti-inflammatory omega-3 oils.
• Get some exercise; it has a surprisingly good effect on your microbiota. Sweat, apparently, is a bonus.

With a healthy microbiota, you will be in the pink of health and have your pick of microbially compatible partners. Treat your love bugs well, and they might help you find "the one" better than any dating app.

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