The Science of Spit

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The human body is an amazing thing. For each one of us, it’s the most intimate object we know. And yet most of us don’t know enough about it: its features, functions, quirks, and mysteries. That’s why we’re launching a new series called The Body, which will explore human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.

Your mouth is a battleground—a wet one. Trillions of microorganisms wage war against each other within. On one side are at least 700 species of bacteria. Most are harmless, and some even beneficialbut a few major species repay your hospitality with tooth decay or gum disease, such as Streptococcus mutans, first described by 18th-century Dutch scientist Antonie van Leeuwenhoek as “living animalcules” because they wriggled around so much under a microscope lens. These spiral-shaped microbes build colonies on smooth tooth surfaces and turn the sugars and starches of food stuck in your teeth into acid that dissolves enamel and causes cavities. But Streptococcus isn’t the only danger. Below the gum line live Porphyromonas gingivalis and the recently discovered Prevotella histicola, both of which cause gum disease.

On the other side are the white-caped superheroes of your mouth: enzymes encased in a watery, frothy substance known as saliva or spit. Your mouth makes two to four pints of spit every day. It’s produced by the salivary glands inside your cheeks and the bottom of your mouth. While saliva is 99 percent water, its constant circulation keeps bacteria from settling on your teeth, and swallowing flushes them into your stomach. (Saliva also performs the important function of keeping your mouth’s pH slightly basic, preventing the acid produced by microbes like the S. mutans from making your teeth fall out.)

While saliva polices the neighborhood, there is also co-existence going on. Between meals, when there’s little for bacteria to feast on, some species dig into the protein in saliva and in the fluid secreted by gums. Having a certain level of microbes in your mouth is important: They boost your immune system and keep respiratory infection at bay.

Which microbes occupy your mouth matters. While 200 odd species are common around the world, many others depend on your environment, race, or food preferences—and they can tell a lot about your overall health.

That's why last summer’s headlines suggesting you toss the floss made many dentists frown.

A medieval dentist at work on a patient, circa 1500. Image credit: Hulton Archive/Getty Images

“Some patients need to floss, others do not,“ explains Dan Malamud, professor of infectious disease at the NYU School of Medicine.

Not only does your particular micro-fauna dictate the importance of your daily routines, Malamud tells mental_floss that saliva holds a lot of information, and at times can be a more useful diagnostic tool than your blood.

Just a drop of your spit can tell doctors if you are prone to heart disease; if your pregnancy may be complicated; if you’re stressed; or, if you're a woman, whether you have a healthy relationship with your father. It may soon also reveal whether you have cancer. Spit is easier to access than blood, too.

While this dark, moist cave is bacteria’s fantasy real estate—an easy place to grow and reproduce—your health depends on keeping them in check. Moreover, the right balance of microbes is important: bacteria on your teeth are being kept in check by bacteria in your gums that produce bad breath. And while the species P. histicola may cause gum disease, it was discovered recently that it also helps control rheumatoid arthritis. And the chemical compound opiorphin, found in saliva, helps activate opiate receptors, which block pain signals from the brain—giving new credence to the idea that you can kiss a booboo to make it feel better.

But there is still a lot that is not understood about our spit. Only half the microbes in our mouth have been sequenced and identified, Xuesong He, a microbiologist at UCLA school of dentistry, tells mental_floss. The Human Microbiome Genome Project, an initiative by the National Institutes of Health (NIH), is currently funding a massive effort to identify all of the millions of microbes that inhabit our bodies.

He and his group recently identified [PDF] bacteria in the mouth called TM7x that live parasitically, meaning they occupy the host and rely on it for nutrition. This is the first time a parasitic bacterium has been discovered in the human body. Its host, XH001, is a “relatively good bug,” He says, but when saddled with the parasite, it may cause gums to become prone to disease.

There may be many other such connections lurking under our tongues. NYU’s Malamud, for example, is currently working to develop a diagnostic saliva test for the Zika virus. A blood test exists, but blood begins to develop antibodies to Zika after 7 to ten days, and then the virus becomes undetectable. In one recent CDC report, researchers were able to detect Zika in saliva up to 20 days after infection. As the virus progresses in stages, its long detection cycle in spit allows doctors to tell when the person was infected. Moreover, the ressearchers were able to spot Zika in saliva 81 percent of the time—a rate second only to urine (92 percent), and a third higher than serum (51 percent).

Malamud sees the sequencing of human saliva as one of many steps toward personalized medicine. He says, “When it’s all put together in five to 10 years, it will change the way we practice medicine."