A new worldwide survey of the human saliva microbiome – the bugs in our spit – finds that a man from La Paz, Bolivia, shares no more microbes in common with his neighbours than with a woman from Shanghai.

Even though humans have been swapping spit for eons, the same goes for 120 people from 10 other cities in North and South America, Europe, Africa and Asia, finds Mark Stoneking, a molecular anthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

He hopes to use the bacteria in spit as an indicator of human migration and contact. This may sound far-fetched, but another team of researchers has used the stomach-ulcer-causing bacteria, Helicobacter pylori, to date human movements, including the peopling of the South Pacific.

‘Unique’ species

Bacterial genes evolve more rapidly than our own, so measuring genetic differences between bacteria from various populations can reveal population splits that occurred too recently to have left significant changes in human DNA.


Yet collecting H. pylori requires a stomach biopsy, no easy task in Europe or North America, let alone more remote or undeveloped regions. Spit, on the other hand, can be easily collected and is chock full of both human and bacterial DNA, Stoneking says.

“We would like to see if we can develop alternative bacterial species as good markers for human migration and human population contact with saliva,” he says.

However, his team’s initial results dampen those hopes. Few geographical trends emerged among the more than 14,000 bacteria that his team identified in human saliva.

The most common constituent was a bacterium called Streptococcus, which accounted for about 23% of the flora. But many species of bacteria appeared to be unique to individuals.

Hunt continues

“It reminds me a lot of the skin in that there’s a lot of diversity,” says Martin Blaser, a microbiologist at New York University School of Medicine. He wonders if additional sequencing will uncover more predictable patterns of diversity between people from different parts of the world.

“The fact that the variance didn’t track with geography in particular was a surprise to me. I would have expected more of a pattern,” says Ruth Ley, who studies gut microbial diversity at Cornell University.

Stoneking hasn’t given up on using spit as a marker for human migration. Additional saliva sequencing could uncover individual species or even specific genes that coincide better with geography, he says.

The key will be to find species of bacteria that pass generationally – from parent to child, rather than from person to person – whether by kissing or other means of transfer, Stoneking says.

Journal reference: Genome Research (DOI: 10.1101/gr.084616.108)