Spit it Out!

Your saliva holds clues to your history and your health

Criminal DNA Swab

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Your mouth is full of bugs. Actually your saliva, which is teaming with hundreds of types of bacteria, is the culprit. Some of these bacteria are helping you out—digesting your food, for example—while others might be causing cavities or bad breath (Altoids, anyone?). But according to a new report published this week, the bacteria in your saliva could indicate how healthy you are, where your ancestors came from, and even act as stand-in fingerprint for identifying you.

Mark Stoneking, a molecular anthropologist in Germany, was researching different human populations, back in 2007, to find out if he could piece together a history of their origins and migration patterns across the planet. He did this by analyzing genetic variations in DNA that he'd taken from cheek swabs. Around this time, he heard claims that there is more DNA in saliva than in blood. Skeptical, he and his research team ran a test. They found that there is, in fact, more DNA in saliva than in blood; but this is because much of the genetic material comes from microbes.

"I got to wondering if we could make any use of the microbial DNA that we were getting in the saliva," says Stoneking, who works for the Max Planck Institute for Evolutionary Anthropology in Leipzig, and co-authored the study, published this month in Genome Research.

Researchers have used bacteria-- most notably Helicobacter pylori, a bacterium that lives in the intestines and that closely tracks human migrations-- to track the migratory history of human populations before. But getting a sample of H. pylori is a pain—literally—since it requires a biopsy. Stoneking says that's when it hit him: "If we could identify bacterial species in saliva that could be used to study human migrations and population relationships, that would be much easier to do," he says.

Stoneking had colleagues in 10 different parts of the world take saliva samples from 10 individuals—120 in total—hoping to find some geographic or dietary link that would prove an easier way to track migration. But what Stoneking found presented more questions than answers. After analyzing the bacteria at the genus level, Stoneking found no geographic or dietary pattern. That means the bacteria in your neighbor's mouth could be just as different from that of someone living across the world.

He's now analyzing the data to see whether there is more geographic structure at the species level, rather than the genus level. "If we find anything promising, the next step would be to sequence protein-coding regions—as is typically done in the H. pylori work—to see if the variation at this level associates with the geographic relationships of populations," he said.

So, just how are all these microbes passed from person to person? "We don't know,"
says Stoneking. "Kissing, transfer from the hands, via food-sharing, breathing the same air—these are all possibilities."

Stoneking's research might also have implications for assessing human health. "The mouth is the primary entrance of bacteria into the body," says Stoneking. "It may be that surveying bacteria in saliva can tell us what is going on elsewhere in the body." It's already been established that certain bacteria in the intestinal tract are associated with certain diseases, such as obesity. Stoneking hopes to find similar clues in the bacteria in saliva. But before researchers can use saliva as an accurate benchmark for health, they'll first have to establish what a healthy person's saliva looks like.

As for identifying you, human DNA in saliva is already being used by law enforcement groups in some parts of the country to track suspects. And Stoneking says that it's possible the microbial DNA in saliva might also one day be used for the same purpose.

So take heed: Be careful where you spit.