Complexity of Spit Revealed
Credit: stock.xchng
Credit: stock.xchng

Saliva tells a lot about a person these days, ranging from ancestry to criminal connections. But drool still holds many mysteries, including what the heck lives in it and why each of us has our own salivary signature.

A new study tries to clean this up a bit, revealing that there is a lot of bacterial diversity living in the moisture in our mouths, both within and among individuals.

First the basics. Some experts estimate that the human mouth contains trillions of individual bacteria, more decimal places than there are in the latest economic stimulus package. The new analysis of salivary bacteria found there are anywhere from six to 30 different genera (plural for genus, the second to lowest level of taxonomic categories) of bacteria living in subjects' spit. The most common one was Streptococcus, the same genus that includes the bugs that can cause strep throat, meningitis, endocarditis and bacterial pneumonia.

Saliva was collected from 10 people in each of these countries: the United States (Oakland, Calif., and Baton Rouge, La.), Bolivia, Argentina, Germany, Poland, Congo, South Africa, Philippines, China, Georgia, Turkey.
Salivary microbes taken from subjects in Georgia and Turkey had the smallest differences among individuals. They differed the most among people who lived in California and in the Congo.

Other common bacteria found in the subjects' mouths included Prevotella (implicated in oral disease), Veillonella (which has the ability to ferment lactate, aka milk acid), Haemophilus (strains of this can cause sepsis and bacterial meningitis in kids), and Fusobacterium (involved in periodontal diseases, and can cause skin ulcers).

Dodgy bacteria

Bacteria are notoriously elusive, biologists know. Many more exist than have been named. Lots are hard to grow in the lab.

In this study, a team led by Mark Stoneking of the Max Planck Institute for Evolutionary Anthropology in Germany found a total of 101 different genera of bacteria in all the subjects' saliva, 39 of which had never been previously reported in the human mouth — despite the fact that some 600 species of bacteria (species is the lowest-level taxonomic category) previously have been reported as found in the human oral cavity.

Worse, the analysis found another 64 genera, on top of the 101, that were genetically similar but for which there currently is no name and which have never been grown in a lab.

Currently, there is limited information available on which bacteria should be found in a healthy individual, but if more were known, salivary bacteria could be turned into a better indicator of our risk for oral diseases and issues, such as oral cancers, periodontitis (a disease that can end in losing your teeth) and even bad breath. Not to mention that what goes in our mouths ends up in our guts, so it'd be good to know about that.

"Since the mouth is a primary entrance for bacteria in the body, and bacteria in the intestinal tract also can influence our health, obesity, etc., there could be interactions between the saliva bacteria and the bacteria elsewhere in the body that are important for health," Stoneking told LiveScience.

Equator factor?

The best predictor for whether the microbes in your spit are similar to those in someone else's was weird too, the researchers found — at least in this study. It turned out to be how far subjects lived from the equator, rather than their age, gender, or local weather conditions, such as average annual temperature or average annual rainfall.

"In other words, a population close to the equator in Africa is more similar to a population close to the equator in South America, than to a population far from the equator in Africa," Stoneking said.

The reason for this is unknown, he said, but it's possible that the actual distance from the equator is not the cause for this finding. Instead it could be some other factor that is correlated with distance from the equator, such as levels of ultraviolet radiation.

Most of the correlation with distance from the equator was due to the sample from individuals living in the Congo, which is close to the equator, Stoneking said. He wants to sample more populations close to the equator in the future to see if the correlation holds up.

Other distances didn't matter

However, no correlation was found between the microbes in one's mouth and how far away people lived from one another, Stoneking found. In other words, populations that lived close together geographically did not have more similar bacteria.

The life in your mouth is likely to be just as different from that living in your next-door neighbor's as it is to be different from that living in someone in China or Tanzania.

"The saliva microbiome does not vary substantially around the world," Stoneking said, "which seems surprising given the large diversity in diet and other cultural factors that could influence the human salivary microbiome."

Many more mysteries remain for Stoneking, who says he wants to learn which species all these bacteria belong to, and whether the microbes found in our saliva vary over time, with what we eat and with where we live as we move about the globe.

The results are published in the Feb. 27 online issue of the journal Genome Research. The research was supported by the Max Planck Society.