In an interview with Knowable Magazine, Floyd Dewhirst says:

We don’t really know the number of bacteria in an average mouth. But there are something like 1011 [100 billion] organisms per gram of plaque — so we’re looking at a large number.

What people usually talk about is how many species are in there. The Human Oral Microbiome Project identified a little over 700 different species of bacteria. (There are also fungi and viruses.)

About 400 of the 700 bacterial species are much more common in people than the others. And were you to take a swab of the cheek and sequence, sequence, sequence until you saw everything you could, there’d probably be somewhere between 200 and 300 organisms. They would be distributed almost on a logarithmic scale, with the most common organism making up 10 percent of the population, the second organism 5 percent, the third just 2 percent and very rapidly, by the time you get to the 50th, you’re down to 0.1 percent of the population. There’s this long tail.

Since we eat and drink, we take in all of the other microorganisms from the planet. A splash of sea water, some dirt on your spinach. Eventually, if you sampled enough people, enough times, every microorganism on the planet could show up in somebody’s mouth. [Continue reading…]